Main problem is they might not contain some of the accessories (which includes cables, screws, and most importantly the case back panel plate) and if you receive it without those you can not return it for that reason without paying for return shipping and paying the restocking fee (not all of them will be missing these but you take a chance -- so buying open box is really best left for times when replacing a MOBO of the same model) -- figure if all or most of the accessories are missing you might wind up paying more by the time you replace them all (if you can even find a back panel plate with the proper layout) but if you already have that specific model and do not need the accessories then it is worth it to save the $$
Not a fan of them. They seem to be missing the backplate about 99% of the time, so you have to to on ebay and buy a new one for $15 or $20, probably from the same jerkoff who stole it in the first place. That by itself is not worth the pain in the ass, though the other parts like cables and screws are easy.
Also, sometimes you do get boards that were returned for a reason. I did end up with one where the LAN adapter was intermittently not working and then crapped out entirely.
So basically, no, I wouldn't buy a motherboard open box. Other components yes, but not a board.