I guess the people who use this service fall into the following two groups

1. People who haven't got the hardware to mine and don't wish to spend the outlay, so they would rather just rent it instead
2. Similar to above, people who want extra khash power at coin launches, etc.

Both situations rely on the coin being mined increasing in cost later on down the line. So although I may get paid 0.004 BTC, a few weeks/months down the line the coins my system mined increased in value to 0.01

If I was just getting into mining this setup would appeal a lot to myself as i'd rent enough rigs to perhaps make a few LTC. I'd then just sit on those LTC until the price increased higher (similar to BTC being $90 a coin and now $600+)