The rationale is...
(1) I mess up the statistics for people using PT/GT against me. Also people not paying attention, who'll think I must have a big hand when I raise 4-5BB.
(2) In games where you have people raising pre-flop 10% of the time (this certainly applies to all 6 max, and 2/4 ring), quite often you limp with something tasty like 67s, only for some bully behind you to raise 3-4BB with A2o or something. Most irritating.
(3) I'm not giving any information away (or any more than I would be by limping).
(4) I build the pot with implied odds hands.
(5) Sometimes I'll steal the blinds.
(6) People may think I'm more of an idiot than I actually am.
(7) Hopefully it will massively irritate everyone

Thoughts? What's the downside? How would people counter this strategy (now that I've err posted it on a public forum, hmm)