To ask if they would like to go out for a drink just kinda has the wrong ring. When they are so on the spot all of a sudden then they need thinking time, non-committal time, just the millliseconds. Many would default say no then afterwards think well maybe i should have, but they didn't have the time, but the date is an easy one for them to go with as it is non-committal and understood by both for what it is.