Personally, I'd avoid them. I use an Incase Sleeve myself and have not had any issues. I'd had a Speck Candyshell on my iPhone, which caused marks even when it was a soft case inside. The MBP will be tough in that respect, however, it can still easily get scratched by the case if you don't clean it regularly (which could become a bit of a pain).
If you treat your MBP with care, it won't get scratched (that includes the bottom). However, if you're really concerned about scratches and damage, a case skin with a hard case is definitely the way to go. I'll let someone else give recommendations for that matter.