Of course, it is your choice what you use, but i would never use rainwater.
Whether you are using rainwater, izvorska voda, tap water or whatever, you are not making as good of beer as you could unless you adjust the salts in your water. Rainwater doesn't have much in the way of salts, which is bad for the yeast and bad for the flavor, among other things. Rainwater would likely not be good for dark beers at all without adjustment. Tap water, in Zagreb anyway, is ONLY really good for very dark beers without any adjustments. Same with most spring water here.
If you are ALWAYS using rainwater, it certainly can't hurt to try some other water. water is a major part of the beer and you should try to use the best water you can. Rainwater would certainly not be the best water you can get.
Whatever water you use, understanding what you are doing with it is VERY important.
If you haven't already, read this:
http://www.howtobrew.com/section3/chapter15.html
In short, though, the spring and tap water we have here is very, very high in bicarbonate, which you can effectively think of as raising the ph of the mash. If you make a pale beer with this water without adjusting it, you will extract more tannins from the husks in your grain, giving your beer more astringency, which can be perceived as a sharper, more coarse sort of bitter flavor. If you adjust the water correctly, you will get a cleaner grain flavor,less harsh. Some people confuse this for a sharp hop bitterness, but it is not the same.
Caclium sulfate, calcium chloride, magnesium sulfate and lactic acid lower the ph and mixing in some distilled water can lower that bicarbonate. Dark malt also lowers the ph.
Sulfate and chloride also effect the flavor of the beer a lot.
Aw, anyway, it is all there in the link. It is worth knowing this stuff. Let's say you are already making good beer. Knowing about the water and how to properly adjust your water can take a god beer and make it a GREAT beer.
Not trying to offend any of you guys. It is all about making better beer,though, right? Rainwater will not help you acheive this in my opinion.