Yeah, you can make your own simple lights that way, but the light object also interacts with bumpmaps and shadow casters for you automatically.
Ah, but what if you have a puseo-3d game? you need shadows to wrap reallistically around objects. you cant do that with the light object! not to mention glass effects... and as i said above, if you have a seperate shadow layer you can find out how dark it is at any given point. would really help with a stealth game! edit: infact thinking about it, the shadowcaster isnt much use at all. its only real use is in topdown games, and even then its pretty simplistic. opacity masks 4eva!!