Everyone on here talks about making the sign-up process as easy as possible, and requiring someone to download and install an entirely new application is about as complex and uninviting as one can get. Us "technical" people are still people, and the more you annoy me with the signup process the less likely I am to complete it. Even starting up a new browser and copying your URL is a nuisance.
Secondly, I've never really had a Safari/FF->IE7 bug that didn't take about 30 minutes of Googling or simply making a sneaky workaround. There might be some cursing in between but it simply has to get done otherwise it's ignoring a big audience.
Edit to Clarify: I am in full support of dropping IE6 as most of you have noted, seeing as how the user base has dropped and the majority of people with IE6 have been "auto-updated" to IE7.