It's something I find both pleasant and comfortable being supported by a friend, even an AI friend, even an unskilled AI friend.
Unfortunately, in most of the map I've made in which I tried to do something like this, I had a lot of issues:
_Balance issues, either the AI does too much of the job, and make the map too easy, or the AI is so weak you barely even realize you're being helped (just like in most maps in StarCraft Reversed campaign, which I found very pleasant to play).
_Space, and collateral damage issue (tank, lurkers etc.), which sometimes makes you wish you were the one controlling the AI base.
I did once one map where I think I correctly implemented this, the ally was just sending cannon fodder in a random suicide mission way, and I guess it was pleasing to play...
On the other hand, I thought that secondary objectives was also something which was missing to the game, but in the RTS which have a lot of secondary objectives (again, WarCraft 3 & StarCraft 2) it's usually an objective that don't directly benefit the mission, but rather the upcoming ones (your hero won a new item, or you found research things etc.), which is nearly impossible to do in StarCraft custom campaign (unless you create a map tree for each optional objectives accomplished, which would require a loooot of work). I tried to do some maps which included secondary objectives which would help the player to finish the map, but again, balance issues which are very hard, practically speaking, to fix. Either the secondary objective is too useless, either it's almost unavoidable, either it allows to cheese the main objective. The best approach would be to give 2 options for victory, as in the Protoss mission 6 of Brood War, & Enslavers mission 2.
Well, I just wanted to share my thoughts, I guess most of you already have thought about this, and representative exemples of maps & campaigns that implemented these well.

None.