I am confused on one point: why do you never buy unit champions for your Goblin units? Or the Black Orcs for that matter? I'm sure there's a good reason, I just can't fathom what it could be! The Orcs get full command... I can certainly understand not getting much value out of a single extra Gobbo attack, but 1) surely it'd be worth it if only for challenges when your opponent's character can inflict 5 extra wounds on your champion and you only remove one model (keeping you steadfast longer, no?), and 2) isn't it worth it for the Black Orcs? Every extra attack counts with a unit their size, doesn't it?
Nowadays there are usually no downsides to buying champions for your units since in 8th edition you can just refuse a challenge while still keeping your champion in the fight (not the same for characters though, who have to retreat to the back ranks). The reason I sometimes don't buy them is to maximise the point value, since they usually come in at the price of about two additional rank and file models, so two additional wounds you might say. Are they worth it?
Sometimes yes, when the champion is a superior fighter compared to an ordinary model for example. This is usually only the case for Orc Boyz, as their champion has +1 WS and +1 S as well as the extra attack, so even if he's not in challenges he actually adds quite a bit to the hittyness of your unit, even though he will cost you more than two additional boyz. As you say at times you might maintain steadfast for longer against a hard hitting character if you can force him to accept your champion's challenge (overkill bonus still apply). I would perhaps argue though that if the battle really comes down to whether or not the character kills enough to remove your steadfast in a turn, you're engaging in a battle you should not be fighting or you've brought too few boyz to the fight. Also, this is mostly for when the character is fighting you on his own, outside of a unit, as most of the time if a character has joined a unit and you call out a challenge, he will have a unit champion of his own to accept it in his stead. Which sort of brings me to my main reason for whether to bring champions in a unit or not.
If you plan on attaching a character to a unit, then you bring a champion to protect him, either by accepting challenges or by issuing them when needed. If you're not joining a character to the unit, you're better off with the 2-3 additional wounds, except perhaps for cases such as the Orc Boyz, or the times when champions may buy magic items. That's my reasoning at least, and as you've read my blog you know I'd like to maximise value and cut corners when possible to squeeze in that extra model whenever I can. So usually when you see a unit of mine without a champion, it's because I don't plan on putting a character there.
In case of my fighter characters, Warbosses or Big Bosses, the champion is there to accept the challenge if the enemy issues one, for two reasons: The boyz are tough and usually plenty in numbers, but they aren't that good at killing stuff, while the Bosses are. To maximise my combat resolution, I want my characters to kill a lot of enemies, which is a lot easier if they're hitting rank and file enemies, instead of tough characters with ward saves. Also, the characters usually provide something very valuable (Inspiring Presence for the Warboss, Leadership re-rolls for the BSB, etc) that it'd be a shame to lose them if you do end up fighting said tough characters with ward saves, and whatever nasty weapon they might be carrying. Sadly, there are quite a few enemy characters of other races capable of beating even our toughest bosses in a straight up fight. So although the lore tells us Warbosses love a good, scrappy challenge, you'll find the army will do a lot better most of the times if the General is alive and providing easy kills for your side.
In the case of your shaman, the champion will usually be called upon to instead issue challenges when necessary. This is mostly for when a Treeman Ancient or monster-riding character charges into your unit in hopes of cutting down your shaman, not so much for block combats. Since the enemy won't have any ranks to hide in, he is forced to accept challenges when issued by the champion, effectively sparing your Shaman for a time and giving you a round to send backup. A nice trick in the new edition then, whenever faced against said Treeman or other Monster character, is to place your champion on one corner of front rank of the unit, and your Shaman on the opposite corner, effectively putting three rank and file models between the two of them (or more, if you're ranked wider than 5 models). When the monster charges in and closes the door to get in base contact with your Shaman, he can only maximise so that 4 out of five models in your front rank touches him, meaning the champion is left out of the fight. So when the champion challenges, and the character is forced to accept, he must move into base contact with your champion, leaving the Shaman out of base contact. Once the champion is killed, the Shaman remains out of reach and the Monster will be stuck killing rank and file for the rest of his time while your Shaman is safe.
And in case of the monster character crying out a challenge before your champion is able to (since the charger is the first one to call for challenges), and your opponent is fully aware of the above consequences if he doesn't, your champion is supposed to move into base contact if he accepts, replacing one of models already there. That's when you replace the Shaman and puts him where the champion was, still out of reach and safe. The second part is a bit more shifty, and you probably risk facing arguments that you should replace the "closest model in base contact" but it's perfectly legal according to the rule book, not to do this. I guess it also makes sense lore-wise, as I'm sure the rank and file would mob up around the shaman to protect him as long as possible. Just a little trick to keep in mind.
So to summarize, I'd say if you got the points to spare, buy yourself a unit champion. Definitely do so if you plan on having a character join the unit, and look for bargains where you can. The Black Orc champion, for example, was notorious for costing far too much for basically nothing but the ability to challenge and the extra attack, in a unit where each model is already quite expensive, so each extra wound will matter. For goblins, this is less of an issue. Just bear in mind that too many unit champions that aren't really doing anything for you will eventually start adding up to one less fanatic, or one less troll, so more than anything I suggest that if you do buy a champion, make sure you have a reason to justify the added expense.
I hope that explains some of my reasons.