Hardly a good comparison since the BG thief can force higher level NPCs to spawn earlier and can surround himself with them thanks to faster level progression. They can jumpstart party XP/progress with thieving skills, can steal nice items from stores, can block doorways with stealth, can backstab annoying casters and can trap everything to death if you feel like cheesing stuff. The bard can do a lot of that as well as cast spells, use wands and has on-demand fear protection with his song.
Even the vanilla versions are much higher on the power tier than your average fighter, as long as you don't actually play them like fighters.
Anyway PoE is much more combat-heavy and in that respect classes all have their specialties as well as pros/cons, so that everyone has the potential to be useful. Some classes are trickier to figure out than others though... the learning curve is an important factor especially for classes/builds that are ability-reliant. In that respect, at least, there are classes in PoE like the BG thief/bard.
Edited by scythesong, 10 May 2017 - 12:15 AM.