I think it's mostly because so many people use frames BADLY, and that
results in web pages that are impossible to navigate in, as well as
clunky, ugly, and disfunctional in general.
If you know what you're doing, you can make a page with frames that works
great for all users. If you don't, your frames are going to infuriate
users and drive them away from your site.
You can get around the search engine problem, but again, you have to know
what you're doing. And a lot of people don't seem to!
If you want to use frames, just make sure you read a lot about how to make
them work well for users, be careful to make sure all your links work all
the time and that people can easily find individual pages loading in your
frames if they need to (because they usually can't link to a page that's
been loaded into a frame and it's frustrating sometimes if you want to
find that page again, but can't bookmark it). Learn, learn, learn what
you need to do to make frames work, and what the drawbacks and other
options are. There are web sites all over the internet about it -- take
some time to do a good job, and you can use all the frames you want.
You know, in my humble opinion.
--M.B.
