i'm also sitting with squarejaw. most IMs are (a) a request for sex/cam/cock pics etc, (b) quite void of content like "u r hot" or (c) a four page story about someone's life.
further, i'm usually busy doing something else instead of watching for IMs and should any come in i'm quite likely to miss them. in addition to that, i already have all my standard IM protocols running in one program. considering that i have about 30 browser tabs up with about 6 web profiles all with their own IM gadget, i would be playing hot potato to keep up with them.
not to mention that most web IMs are java clients and are poorly written so they eat up memory and cpu. the web is already a cesspool of horribly written websites that are bandwidth and resource intensive (myspace has got to be the all time #1 of these).
if someone wants to IM me, they can hit me up on aim/yh/msn/icq/irc and oh _please_ say something intelligent instead of one letter words and three letter sentences.