One reason to upgrade would certainly be the support, sustaining support is indefinite but that's got a lot of limitations compared to Premier or Extended support.
V6 has just as many bugs as V7, you probably just haven't run into them.
For me the best reason by far is to be able to take advantage of newer technologies, even if you aren't looking at the Enterprise features like Web or Team Member your IT infrastructure personal will want to upgrade your database server or the operating system at some point in the future but they will be hindered by what a product released 5+ years ago supports.
As far as adoption rates, we recently ran a user group with about 70 users attending and we directly asked them the question of who is on v6, v7 and v8.
Results - zero on v6, about 60% on v7 and about 40% on v8.
I do personally know of at least 2 customers still on v6 but that is out of a customer base of 200+ customers.
Time for you to come out of the dark ages