Namecoin isn't a currency. Cost is designed to decrease, since economic scarcity is provided by the limited number of unique IDs in the namespace of the TLD instead of the limited number of answers in Bitcoin. The "coin" itself has no intrinsic value on the Namecoin network, only the name has value.

Namecoin isn't designed for people to "compute4cash" at home, or use it as a financial tool. It's designed to make sure governments can't arbitrarily seize domains.

Namecoin's critical flaw, in my book, is that it doesn't satisfy human greed in the same way that Bitcoin does. Namecoin doesn't enrich the miner, where as Bitcoin has a built in notion that it "earns" you money.
If you're looking to "get rich" from your "idle computer time", stick to Bitcoin. If you've got an idealogical issue with governments seizing domain names and wish to see registrars and DNS decentralized for the health of the Internet, then support the Namecoin effort with a few Mhash.