The big problem with using a full-size drill for such purposes is, obviously, that it's so BIG. Depending on the particular situation, that may or may not make much difference. Regular drills don't turn as fast as Dremel Moto-Tools - but, as we've established, in most modeling applications that doesn't make much difference.
There's another option. At least one dealer, Micro-Mark, sells a chuck attached to a hexagonal shaft that can be chucked up in either an ordinary drill or an electric screwdriver. Most electric screwdrivers I've seen offer only one or two speeds, but they're plenty slow enough for work in wood and plastic. And they aren't much bigger than a Moto-Tool - and they cost considerably less.
I suspect, though, that most modelers who've invested in Moto-Tools are glad they did. If you can work out a way around the "too-high speed" problem, the versatility and handiness of the "real thing" are hard to beat.
I can now add what I think is a rather interesting postscript to my own experiences with Dremel tools. I think I mentioned earlier in this thread that my twenty-year-old Moto-Tool (actually it has a Sears Craftsman label on it, but it's quite obviously made by Dremel) had given up the ghost. The chuck had suddenly stopped turning - though the motor was still running. I concluded that the drive shaft had busted, and that it was time for a new Dremel.
Fortunately I didn't throw the old one in the trash. When I'd had time to think about it a little, I realized that a broken main shaft was highly unlikely. I took the thing apart (by unscrewing the nosepiece and removing six Torx-headed screws) and the problem became obvious: a small plastic (Delrin?) coupling, between the chuck and the motor shaft, had shattered (probably due to simple old age). I jotted down the model number (Sears Craftsman #572.610030), and the next morning I typed that number into my computer's search engine. That got me a link to the discussion section of the Dremel website, where, lo and behold, another customer had related an identical story. He even included the replacement part number. So I phoned Dremel customer service (the phone number is on the website), and talked to a nice lady who put a new coupling in the mail (along with a metal spline that, she said, Dremel recommended replacing along with the coupling). Total cost, including coupling, spline, shipping, and sales tax: less than $5.00. Bravo Dremel!
But I stil wish the company would make a small, battery-operated rotary tool with a speed range of, say, 0 to 2,000 rpm. Until it does, I'll continue using my old Sears beast for heavy duty jobs (like grinding and routing), and my little We-Cheer "compact power carver" for most drilling and similar work on models.