The way I see it is for each shared tree there is always one class that does it way better.
Rage is better for jug
Madness is better for sorcs
Pyro is better for PT's
Lethality is better for snipers (just what I was told)
There is nothing wrong with going doing the shared tree for any AC but some classes just have better tools/abilities to make it that much better.
Also, some shared trees are viable for both ACs...some not so much. Madness Assassins are extremely rare, and having been playing since early release, I've seen exactly 1 (ONE) lethality operative. The shared trees for those two classes are simply not really viable, unless you are doing it just for fun. The lethality one in particular, you're actually going against your core mechanics (stealth, heavy opener from stealth, need for TA) to use. Requiring a 3-4 ability set up to use a channeled ability from melee (with no real gap closer)? No thanks.
OTOH, Smash Marauders, while in some respects less potent than Juggs, compensate in other ways; and Pyro mercs are at least better than arsenal mercs, even if they still underperform compared to pyrotechs