On the PSR740 the mic input is 50k unbalanced and uses a cheap mic amp (LM358). It's also filtered quite a bit - 10db down at 10kHz, so it's not a serious attempt at fidelity. Yamaha market these keyboards as home keyboards. I think the average user wouldn't know XLR from the rear end of a horse. All the folks I know (present company excepted) are amateur home users who just want to play a tune or two and probably never use the mic input. I don't use it either as I can't sing anyway though I sometimes use it as a line input.