Well the file sizes are not 18MB The camera is 18MP but going from the Canon web site, the RAW file size will be 24.5 MB. That's a huge difference from a 10-12 MP file at under 15 MB. Load 40 or 50 of those in Lightroom and see how fast your computer runs.
Sure memory and hard drive space is getting very cheap these days. But not necessarily good quality SD cards. Extreme III's and Lexar pro's are still up there for those smart enough to use a good quality card.
Lets say you had to do all 3.
$100 for a new hard drive
$50-60 for 2 Gb of ram
$100+ for a 16 GB SDHC Extreme III
(B&H)
now lets consider that your computer is old and just lags like crazy on 24.5 MB files. But you bought up to a DSLR for the RAW IQ. You system can't be upgraded because the motherboard is too old or already full at 2 GB. I have one here like that which I'm typing from right now. It's fine for web stuff and almost anything else but forget editing big files. It's just too slow and it's maxed at Duo Core 2.0 GHz and 2GB ram.
So I have another system for editing. But if you didn't then you are looking at another $750++ for a faster CPU and much more if you are using an Apple or want a powerful laptop.
there's more to it than just the camera.
So suddenly your cheap camera is $250-300.00 more than you anticipated. maybe it's going to be $1000 or more for a complete system upgrade. For an experienced shooter, this might be expected and they may have already upgraded the hardware they need. But if you have been shooting a P&S and editing 4 MB Jpegs, they may not have considered any of this. That was mostly what my post was about.