Actually, I did this last night with a bud for a little while.
The results were mixed, and I think the addition of an attachment point would lead to abuse.
When on my bud's mech, all was fine when I "crouched" when on him at full speed....right up until he hit his jump jets.
At that point, we just looked damn silly as he'd jump, and fall at a faster rate than me as a BA, so I was kind of like a top hat falling on a cartoon character a second after he made contact.
Also, the ManpackPPC knocks you right off when firing, etc.
I encountered maybe two times it was useful for him, rather than me so much, to have a set of eyes to see the incoming Aeros and enemy BA's. Oddly enough, I just wanted to see how useful it was for me to TAG for his LRMs.
There seem to be enough detractions from doing it all the time, as you end up more useful when not riding them I think. It didn't take long for me to figure it out.
While the claw/attachment point sounds cool, I can see an Atlas with like 4 BA's hanging like Christmas ornaments, rolling out, unloading at once, with BA's mopping up and other scenarios of abuse.