Oh right. I get ya. In the UK they are getting much cheaper and places like Toys R Us or Game Station usually sell them for fairly low prices. I guess it's different in other countries.
I only have a couple of gripes against playing GBA games on the DS. The first, aside from the single black screen, is that you get a small black border around GBA games that aren't there on a GBA. And the second is that when you stick a GBA cartridge in a DSlite (which is what I have) it sticks out a little bit and, uhhh, it kinda irritates me.
But I'm just being fussy!
I had the GBA SP for a long time so thats why I just stick with that for GBA games. Might get a micro soon too as the screen is just beautiful. But I see your point too. If you don't have a GBA and it is still pricey in your own country, then yeah DS is great for GBA games.