I think the problem with Sparkgear was that they'd never faced treants before (and if you don't know how to handle them, they mess you up real bad).
Wrestler treants strip dwarves of their equipment, including their weapons (if you don't use blunt weapons), and furthermore, even if you mass dwarves, if at least a few aren't of elite or champion level you're probably facing significant casualties.
Their main strategy (in the older versions) is to crowd a dwarf, tank him with their great size and damblock, perhaps steal a weapon or some equipment. Without backup, eventually the dwarf will become overexerted and suffer a broken limb, at which point he will cease fighting and attempt to flee, and is essentially dead.
Thus, treants are dramatically different from goblins, or even orcs, and fighting them well requires preparation and strategy, which in turn can only come from experience.
These are challenges that a mature fort can handle, and even welcomes, but a disorganized, community fortress, inexperienced with Relentless Assault, which also suffered from little to no continuity between leaders apparently was unable to mount sufficient resistance after being caught off guard.
Personally, I like the variety that this kind of enemy adds to the game, which is why I leave my treants this way (I think I upped their size to 14).