Sometimes, the inevitable happens. You're down a man (or woman), halfway through an instance, and one of you has to pick up a replacement. Or perhaps you're on the other side -- you really want to get into a particular instance to do all those quests cluttering up your log.
Some players in the latter category may hang around capital cities occasionally asking in LFG and waiting for appropriate LFM broadcasts. However, it's safe to say that not every player in Orgrimmar wants to do RFC, and not every player in Stormwind wants to do the Stockades. Unfortunately, I've seen a huge rise in the number of unsolicited whispers recently from players asking for help in these instances -- and more.
I wouldn't mind so much if they were phrased nicely, or gave me some motivation to join. "u want 2 come to dm with us? u heal?" isn't sufficient justification for a high-level character to leave the comfort of a city and run through a low-level instance with strangers. Of course, some players take this opportunity to play tricks on unsuspecting lowbies -- what's your reaction to random instance invitations? Have you ever found one compelling enough to take up? Or did you manage to phrase your LFG nicely enough that you were deluged with help?