Do any of you do this?

Aside from estimating the time/month that would be needed, it strikes me as a way to be a 'kept' person with a loss of freedom to do other work because the person paying the retainer will expect me to put them at the top of my list and come running if they have a little problem. I have a client who wants to set up something like this- I haven't thought about $/month yet but I would rather not get into a situation where I go there to do little piddly crap just to satisfy the retainer. On the surface, it looks like some easy, regular income but deep down, I think it could be treacherous unless some strict ground rules are set up.

Opinions?