Protein during IM?

Not necessary at all.

Some like to use a little because it helps make them feel more satiated versus just taking carbs all day.  Taking too much tends to cause GI issues for many.

On my first century I stopped for a burger.  very satisfying. 

not necessary but if you don't get protein during an IM you'll feel hungry most likely and also there are studies that show increased muscle cannibalization w/o protein since the body needs it in efforts typically > 2hrs.

I have found on my longer training rides, if I take in protien via a drink like accelerade, I have some GI issues right after I'm done, which could really be a problem if I had a long run next.

I have found sticking to just carbs with GE or some other drink, that DO feel hungry, but without any issues.  I have taken in a harvest bars on occasion with no ill effect.

But I'm still experimenting during my training for my first IM.

I have a friend that did IMCdA and stopped on each loop at Subway for a 6" turkey sandwich

I like others have an Infinit mix with protein for the bike, not really necessary but helps satiate a little.  Does make you fart though.  For the run I have no protein because it seems easier to digest once the HR starts rising on the run

I used Perpeteum for IM FL.  I had no issues with it during training.  However, on race day I had reflux and burping issues on the run.  My stomach didn't feel so great the first few miles of the run.  I solved the problem by drinking mostly water for the first few miles of the run and that seemed to solve the problem and I felt great after that! 

I would say, go for BCAA and not for proteins (eventually). You need some time to assimilate proteins while BCAA are very fast and will be used for sure by your body.

Try to train with BCAA, before starting, in the middle of the cycling and after training/racing.

No problem at all with digestion. 

That's why it's in "recovery" drinks.  Good for post-race, not during race.  There's no need for protein from a performance standpoint.  But some people find that some protein (can still come in liquid form) makes them more "satisfied", which can be helpful over a long IM day.  Personally, I'm fine without it and stick with sugars (carbs) all day.