Back in the early days of commercial web design, the web design companies would either charge their customers for every page that they created or the ones that were more results focussed would charge for every hit they received in a certain time period. Both of these charging methods led to more pages being created than were necessary and hence the idea of an intro page was invented.
The intro page was another page that the first group of web designers could charge for and it also meant that every visitor would have to view the intro page, plus the page they were actaully looking for, so they were paid for two hits instead of one. The idea behind intro pages was never to make the site more professional or to serve any other purpose than to earn a commercial designer more money.
The only purpose of intro pages serve these days is to annoy your visitors. You visitors want to find the information they are after as soon as possible and wasting time looking at your pathetic intro page is only going to annoy them more. If they are using a dial up connection, they are not going appreciate the time and bandwidth that you have wasted for them. It is not a very good start from which to try to sell them your products or services. Given that research shows that you have less than five seconds to convince your visitors to stay on your website, adding an intro page to the equation is only going to reduce the time available to you. There are absolutely no plus points to having an intro page. All they will do is alienate and annoy your visitors and those that aren't annoyed will just use another site anyway because you've left yourself with no time to convince them to stay.
Another bizarre scheme these days is a flash intro. Just in case the idea of wasting your visitors time with a static intro wasn't bad enough, some people have to waste their 5 seconds and then take up more of their visitors time demonstrating how pathetically sad they are that they haven't got anything better to do than waste their time creating flash intro pages. Visitors are not interested in flash intros, they want their information. Give it to them, for Christ's sake, and stop wasting their time and bandwidth. The websites that have the audacity to not include a "Skip Intro" link are even funnier. How many salesmen have you seen knock on your door and then prance around looking like a moron for 20 seconds? I'd be willing to bet that they'd sell nothing. And neither will you, so don't do it. Read my Flash and Shockwave article to find out what else you shouldn't be doing with Flash and Shockwave.