Maybe it's the girls I pick, or some effect I have on them, or it's key to the female psyche, but nearly every casual long term hookup I've had has ended with an "I can't do this anymore, date me or we're done but I don't want to be done so just date me". Usually some drama/tears/hooking up then I'd either commit or leave. Not really the original intention at all. Not fun.
I think the only way it can really work is if she sets the terms from the outset. Junior year of college I had a girl who would text me at 2am, in no uncertain terms, to come over and.....deliver. She knew exactly what she wanted and I enjoyed it. Not all that fulfilling in the end. Sounds like an ideal, but left me feeling a bit used.
Ultimately I'm not sold on the practicality of a long term friend-with-benefits. I think a lot of what mscriv said should be well considered.