Here's the deal with single renders: RAM
Let's say you've got a map that is represented by (128 x 128 tiles, 32x32 pixels each) 16,777,216 pixels. If this were 32 bit, that would be approxamately 67 megs of data. If you want to do fancy engine things such as layers, well, you can add that 67 megs.
Even someone like mouse should be able to see that ten of these maps wouldn't be able to fit on a single CD-ROM.
If maps were built out of tiles that were re-used, then the amount of space data required to re-create a map for the player lessens. Big chunks of it are re-used.
Each map is likely stored in RAM, this is simply the best way of doing things. Coldstone will need to keep track of other things besides your map, such as any animations, sounds, and music you want to have playing, and these all require RAM too. Either that, or Coldstone will be accessing the hard disk quite a bit. Don't forget engine overhead, either.
This all adds up pretty quickly. Like I said, I've got three-quarters of a gig of RAM in my powerbook, but I don't want to limit my audience to myself. That, and I'd like my game to fit on a single CD--I find Multi-disc games too annoying.
Now, granted, by the time any of us actually COMPLETES a game, well, the computing landscape will be quite different. I'll probably be using a Powerbook G5, and the G6 towers will just be coming out around then, shipping with a gig of RAM and OS 11. Games will be distributed on DVD.
But still, there's something to be said for efficiently using resources. 
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People who claim the sky is falling obviously aren't aware the earth is falling, too.
