When you go to a site, you're obviously not there to see the content. What's the point of that when you can see a pretty splash page with a huge enter sign and hundreds of shiny glittery buttons? You don't care if you can't actually get into the site! All that matters is the splash page - it's what keeps you coming back. [/sarcasm]
If you haven't figured it out by now, splash pages are pointless. Here are a few examples of the effect they can have on your visitors:
Visitor A: *taps fingers on desk* I've been waiting for this site to load for 20 minutes now, and I have a cable internet connection! This is ridiculous. *tumbleweed rolls by* Finally! A 900x600 .png image just loaded. Now I just have to wait for 25 link buttons. *the season changes to summer* Yay, it's done loading! WTF, I have to click enter to get in? I've been waiting forever for this, and it's not even the actual site? #(*% this #(@$!
Visitor B: Hmm, this site seems to have a lot of requirements to enter. *reads* Love for my site, RESPECT, no haters, no flamers, no spammers, love for verdana size 8 bold, love for Johnny Depp, love for MCR, 1024x768 screen resolution, Internet Explorer... ahh, I give up! *scrolls* This list of requirements goes all the way down the screen on my 1280x1024 monitor. I'm never coming to this site again!
Visitor C: Woah, there's a lot of link buttons here. *counts* This site has 65 affiliates, 28 hostees, and they joined 13 cliques and fanlistings. This affiliate has a cute link button... maybe I should visit that site. *2 hours later* Finally done visiting all those links! Too bad I can't remember the original site where I found them all.
There are a few instances when splash pages are okay. For example, if your site is a collective with links to your other sites, that's fine. But most of the time, there is no point, and it would be much easier for your visitors if you just skip the splash and put your layout coding on your index page. If you have a splash page with a long loading time, you're probably losing a lot of potential visitors.