Mining contracts only make sense if the person selling the contract is more or less taking the person buying the contract to the cleaners.
For example, say you want to rent 1GHash/sec. Vladimirs prices make sense for Vladimir, you lock yourself in for a multi-month contract at a price that allows vladimir to buy 1GHash/sec of generating equipment, he sells you that 1Ghash for a few months, you get the BTC and he gets shiny new hashing power for free (less power costs) once your contract is up guaranteed (for him).
The only sense for the buyer is if they are in a place where electricity is crazy expensive, computer hardware is crazy expensive, etc. etc. is very expensive, and they expect the price of bitcoin to stay relatively flat, or to decrease, at a rate that is still a profit over what they paid for the contract. A sort of minimal gain but without any hassle on their end. The lazy mans way to get into bitcoin.