You can buy eth if you believe in the future of the platform, if you want to experiment with the network, or if you want to develop programs for it.
I thought that Ethereum = bitcoin +smart contract network.
That sounds more like this:
Ethereum is its own network. If Bitcoin collapses tomorrow Ethereum will not be affected.
So the functions of Ethereum can be replaced by Bitcoin. We have to make Ethereum big and attract more user base. However, it is still smaller than bitcoin at the moment.
If the developers stop dumping, the price might rise and attract new users.
If you need ethereum, then the price is far from unreasonable. But I wouldn't put anything of value on the network as it is. It needs to mature. The user base should come when(if) the mist browser is available to ordinary users. The biggest problem at the moment is that the devs held too much of their funds in a volatile asset that just happened to take a nosedive.