Any time you have woofers covering a less strenuous or broad frequency range then there is an opportunity for better clarity and lower distortion
To give the relief to the mains you mention; doesn't this always necessitate sending the signal through the added stage of the crossover in the sub's amplifier section? Would that also not pose some risk of somehow affecting that signal by sending it through added cables and circuits? The alternative is to send the pre-out to both amp and sub amp (requiring two pre-outs or a splitter), but then wouldn't that negate the effect you are describing of taking some of burden off the main speakers to push the lower frequencies? I'm just curious, because I've heard this very supposition before and wondered about it.
Marco