I think there are both good and bad points. If you have a really homebirth-supportive GP who is hands off, it can open all sorts of doors for you - for a start, it makes it much easier for the midwives to organise certain things, like blood tests if you have a rhesus mismatch or other niggly things that can result in a really unnecessary hospital visit. I don't really know how useful they'd be during the actual birth - I'm wracking my brains to try and think of anything a GP could do that a midwife couldn't, and I can't really think of anything - maybe give syntocinon if you were one of the very unfrtunate few to have a post-partum haemorrhage, but most midwives carry this and will give it in an emergency anyway, with or without a medical order. It's basic midwifery practice.

The only other benefit I can think of is that if you transfer to a hospital where your GP has visiting rights, he can remain your primary physician.