I'm sorry I didn't explain that too clearly. OK, let me do this the right way this time :)

We'll use some examples, as it's often the best way to demonstrate a scenario:

Let's assume that Blackhat #1 has domainXYZ.com and they'd like a link from your site because it has a pretty PR5 that they're just dying to capture.

So, they add a link from some crappy PR0 page on their site to point to:
YourDomain.com/redir.php?site=domainXYZ.com/crappyPR0.htm

The SEs will follow that link to your redirect from their site and end up right back at their site because of your redirect. Now, it looks to the SEs like your pretty PR5 page is linking to the crappy PR0 page on their lowlife site.

This can create problems for your site because once one lowlife blackhat or hacker finds out about it, they pass it around to one another like it's the candy for the day.

Following that, the SEs will look at the amounts of links your site has to all of these *cough* quality *cough* resources, and they'll think ... hmmm ... looks like s/he's into linking to bad neighborhoods.

You'll then get to find out firsthand what the bottom of a SERP feels like, and you won't be comfortable, trust me.

The Internet is full of a lot of freeloaders that want everything for nothing. They don't care that you've worked hard on your site, nor do they care what their antics would do to your reputation or your business. They do it because if their pages get high enough PR, they can sell links and/or ads to other scumbags that are just like them.

I've seen it happen time after time and the blackhats/spammers/hackers NEVER change. The only thing that changes is, their victim, on a regular basis.

I hope this helped to answer your question a little more clearly now? :)

By pulling the allowed urls for redirect from a database, it gives you 100% control over every aspect of it. That kind of control cannot be stolen from you by someone looking to make a PR score off of your hard work.