This is the most irritating thing, they rarely get a reply to this unless I have generally forgot/got lost in the messages, AND it's been a reasonable amount of time. The other reason is if there question requires a genuinely quick answer. But on the whole I find it incredibly rude, if you really want a reply, it is easy enough to put a few words together than just the solitary question mark. Rude and inpatient, it's the worst and a massive reflection on themselves and their character. I would hope that they do respect we have lives and other things going on.