A flexible underlay will compact when walked on. This helps if your floor is slightly imperfect (it is) and people walk on the tongue-and-groove connections (they will). The extra stress could cause the connector to snap off one of the boards.
A more compelling reason is that any slight dips in the floor will cause an audible sound every time you step on it and it bangs onto the sub floor thanks to your added weight.
I'm not aware of any "serious" reasons to use underlay (fire safety, structural, etc). If it's your house and you don't want to lay underlay, you won't be the first homeowner not to bother with it and I doubt you'll have problems.
I always found, however, that a roll of Tuck Tape and a couple of rolls of foam underlay was pretty insignificant compared to the cost of the flooring itself, and the ease and speed of installation was similarly not a serious issue.