Long time ago, when internet was slow and browsers were stupid, we could see a lot of specially crafted pages which welcome you when you hit the website for the first time. At that stone age the splash screens were necessity: there was no other way to ask visitor which language s/he  prefers or whether to direct to low-weight or heavy content depending on the user’s connection speed. Also, users behavior was different: we opened each page as a photo-album or a glossy magazine: the cower was important part of the content, the prelude we’d like to enjoy, the overture user like to listen.

A lot of water has passed under the bridge since then, We need the info faster and faster but recently the splash screens became in trend again. Unlike great-grandma’s dress, it looks really annoying and stupid. Here’s reasons to use it (a while ago) and why you should not use it now:

1. to show that the page is being loaded. The main reason nowadays. Hey, just throw away all what is not needed, tune up server, leverage cache and make it fast! Visitors will appreciate it much more than moving lines/dots/pots. Moreover, half-loaded website is often complete enough to select the next page we want – if we’re still on our way to wherever we want to be – whilst we should sit and wait until that nasty splash disappears.
2. as an aesthetic complement to the main page. Really? Seriously, do you believe that a user comes to a shop page, corporate website or to a blog about gadgets/kittens/travel/websites/kitchen to get an aesthetic delight??? Don’t lie to yourselves: 99.99% visit your page to receive an info (+perhaps to buy if you have a shop). Get it right: appearance is important, but it is not the goal – for most of us.
3. to insert a flash animation (like above but even worse). Are you still using that middle-age technology? Poor Yorick, I pity you.
4. to direct users to the appropriate website for their country or language preference. Use browser’s ‘Accept-Language’ header and GeoIP instead.
5. to direct users to a low-bandwidth site. It is quite easy to get processing time of the very first “text/html” item, so you can detect it.
6. to inform visitors about site requirements such as used browsers, screen resolution as well as used Flash, Java, Quicktime etc. “We require IE5+ and 800×600 or higher”, yep. See: Google(+YouTube), Yahoo, Amazon, FB, Twitter, whole Wiki, OpenStreetMap, GitHub, Bloomberg, CNN, BBC and many more – ALL of them feel well without those “Flash, Java, Quicktime” – just using HTML5 and JavaScript. If your developer/designer  insists on any of this – 99,9% you need another dev/des. If s/he insists you need that technology on the main page – you 100% need another dev/des.
   
7. Visitors can select the preferred view mode – for instance, standard mode and fullscreen mode. Add the  ‘fullscreen’ button in the proper place instead.
8. Multiples sites share the same domain. If domains aren’t cheap enough for you then make informative main page – with correct navigation.
9. Splash page is supposed to include hints for browsing the site and explains the main sections. “Hey, you should use arrows on keyboard only!”. If it is not a game then think twice or even thrice. Shouldn’t it be obvious how to use your website?

There’s some cases when it can make sense to use – not a splash screen but rather pop-up div with shadowed background:

1. as an additional form of advertising. if it not the main/landing page of the website and you want to draw attention to super-discounts you have right now. Nevertheless I’d rather like to see a banner than something which hinders me to find what I’m looking for.
2. to direct user to page which is more accessible to disabled users. Nevertheless, use large enough fonts, build menus so it can be reached with ease when the page is being read out. Consider more contrast theme, put control elements to switch to page for disabled users or to change font in expected place (top right corner)
3. to restrict access to content such as pornography, alcohol advertising or sales, or gambling. Probably, the only case when it really makes sense.

The last nail to a coffin: in some studies 25% of visitors left a site right after seeing a splash page instead of heading into the website itself. That is a large number of people who have just abandoned your company. (source)