I doubt spirits wish to be servants for the rest of their experience in this plane. If you was a spirit, would you? Besides, it would be wrong to keep a spirit as a servant. Its rude to summon asking for a servant anyway; you will just end up loathed.
Brownie Fae are NOT servants, they help out in exchange for offerings of bread, honey, sparkly objects. They dont help out for free, and if you dont leave them offerings you will find they will leave.