some tools will throw out misleading errors on 'invalid' markup, and you will have to spend time justifying to clients why these invalid markups are OK. accessibility tools might have problems (not sure now, but I remember some tools years ago having problem on some invalid markup).

Not likely, few clients even know what markup is. I'll take the extra traffic and suffer the occasional nosy client any day.

I've dealt with state agencies and higher ed depts that get their dander up over this sort of stuff.