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