Is memory usage really that big of a deal this day and age?
If you don't mind spending money on it, no. If you have better things to spend your money on, yes.
I can quite confidently say that I regularly have at least 5 apps running. Out of 4GB RAM they usually take up in total about 400MB. Out of 4GB RAM I have installed, that's not bad IMO. However, the browser is almost always the biggest consumer of RAM - if they were all like that, and if I only had 2GB, I'd be out of RAM and system performance would be impaired.
"RAM doesn't cost that much", you would probably respond. UK prices:
2GB DDR3: £11.99
2GB DDR2: £31.19
2GB DDR1: £46.79
So the older the computer you've got, the more it costs, making one less likely to upgrade a part of their computer and possibly consider putting off the purchase until they can afford a completely new computer.
Now, use your logic of "system resources don't cost that much" to CPU, HDD and graphics. After all, the latest mainstream computers have a lot more resources at their disposal. Suddenly it becomes more cost-effective to upgrade one's whole computer about every 3 - 4 years, just for web browsing.
This isn't really rocket science. Simply don't assume that everyone's priorities are the same as yours.
I bet in light of tablets and netbooks coming along, companies like Microsoft are wishing to hell that they had made resource-efficient computing a higher priority. Instead MS had to keep XP going until Win7 came along, and from what I've seen, Win8's memory footprint is half of Win7's.
Resource-efficient computing means greater flexibility. Who doesn't want that?