If I were a newer or less eloquent contributor, I'd certainly worry
that MY pet topics might get deleted, that's for certain.
I don't think that worthy articles get deleted very often at all, but
the issue is as much that occasionally, there's a big ugly fight over
something that really should not have been up for deletion at all, or
should have been speedy kept. It's not helped that the Wikipedia
community has a circle-round-the-wagons mentality when faced with an
angry newcomer who doesn't know how things work. We tend to
reflexively want to do things against those who come here and tell us
what to do.
There are quite a few reasons why those interested in an obscure topic
would be BETTER off at Wikipedia, though:
1. Anti-vandalism assistance. Vandals can utterly destroy a small
wiki with automated tools. Natural responses to such vandalism -
including making it hard to edit - effectively kill off the project
because no "new blood" gets in.
2. Writing assistance. Even if I don't know anything about your
topic, I can help you with grammar, presentation, etc. Wikipedia is
big enough that such help will come along.
3. No hosting worries. If you start your own wiki, you have to host
it, you have to be sure you can pay for it, and you have to be sure
it'll stay up. If you put your stuff on Wikipedia, it's pretty
certain to remain available.
4. Prominence and advertising. You're much more likely to find new
contributors if you attach your project to a top 20 website.
There have to be a few more, too. Anyone think of any? We might want
to have a Wikipedia: page about why NOT forking is best.
-Matt