Amino acids are fast acting, and absorbed better than whey anyway. If you're doing silly long sessions, BCAAs may help, but remember they're highly insulinogenic (create a big insulin response). I still think a banana is more effective during a workout, however, BCAAs won't be a waste of time really either.
BCAAs contain things like leucine which helps initiate protein synthesis - which frankly, won't happen during a workout... however, the caveat I will say, is that loading up on BCAAs during a workout may not enhance your workout, but potentially could help reduce DOMS and improve protein synthesis post workout - but as I said, you've got a good 24hrs post workout to eat, but may also serve as way of converting energy too - but my biology is a little rusty.
You won't be doing any harm taking them during the workout or taking a step backwards really - certainly if you're training at such a high intensity.