My turn!
Underpaid, yes. Unappreciated, certainly. But then the front line of most industries tend to fall into that trap because people are people. Get one person who can't do the job right and it tarnishes the whole desk. Get one person who complains a lot because they can't get their own way or because they can't put their problem across properly and the same thing happens.
I've been on both sides of the argument and I've seen the problem at first hand. The helpdesk faces the outside world where there are invariably people who are unfit to have a computer but who will argue the toss at the drop of a hat and will complain loudly when they lose the argument. At the same time, however, there are those helpdesk folk who are little different to that and, given that many companies treat the helpdesk much as one might treat the family cat's litter box, it means that this is likely to continue.
This is complicated by the fact that many places that operate the entity known as a helpdesk, or "Service Desk" as ITIL says I should call it, have no real notion of exactly what it does and spend little time or effort finding out. A typical gripe, for example, appears further up this page in which the staff are referred to as "poorly trained" but how many companies out there are committed to giving their helpdesk staff the training they actually need? And, for those helpdesk folk out there that might be reading this, how many times have you ever been asked a support question about something implemented without your knowledge? I know it isn't supposed to happen in a well run IT department, but... really...
Even ITIL itself is non-committal about the whole situation, calling itself a "framework" and allowing for adaptation when, in fact, this is a licence for ignorant managers to simply run roughshod over the entire principle when occasion fits. Believe me, they do! "I don't care if the policy/procedure says that, do it now!"
Add this to the usual "my job is more important than anyone else's", "I'm a VIP so deal with it or you're history", "you can do it this once for me, pal", "honestly, it isn't my fault; I didn't do nuffing guvna" and you begin to wonder why anyone in their right mind would ever become a Helpdesk operative!