I find the ones who ask this (or send other vague, pointless texts - are you working today and so on) almost always turn out to be the one who are expecting to turn up in the next half hour.
It's infuriating, and whilst I don't take short notice bookings anyway I know other people do, so leaving the lack of reading comprehension aside does it really not occur to them that time might be a factor if they text at 1.40 wanting to book at 2pm?