There is currently every reason to just get the fansub instead of buying it...
I started with fansubs because when you were in high school and early college, you don't really have the money - especially not when a box set cost $90 to $120 USD. Yeah, the prices have come down considerably now, but even at 40 to 50 USD, you've got to realize those shows are 5 to 10 years old now.
That brings up another point - delay in releases. I'll watch the fansub, for example Burst Angel / AKA Bakuretsu Tenshi - it came out in 2004, I watched it at least 2 or 3 years ago on fansub - and only about 6 months to a year ago did I see it in the anime magazines "Burst Angel the next big anime?!?" That'd be like for all the people that somehow enjoy watching 24 and American Idol having to wait a year to find out what happens. "Hey guys, did you know Jack Bauer is in exile?" "Dude, that episode was like 2 years ago."
The 3rd big issue is quality. Fansubs are generally made from ripping the show from over the air TV that it airs on in Japan and add the subs. Every time I've tried "doing the right thing" and going and buying the DVD box-set, the DVD version looks like shit compared to what I downloaded a year or 2 ago. And I mean recently, within the past 3 months, where HDTV's are more common than ever. Also, as many other people have said, the English voice actors on dubbed DVD's tend to have retarded voice pitch, no emotion, and FUBAR the timing on the lines
I'll end my rant now, being as my lunch is over and I've gotta get back to work...