I am not for guests in MK games. I feel that the inclusion of guests makes this more of a Capcom vs SNK/Marvel vs Capcom/etc. type of game and MK is not like those other games....now, I love all the VS games so this isn't any sort of hate for them, I just prefer my MK to stay MK.
With that being said, I still don't make a huge deal out of guest characters. I would much prefer MK characters to fill the roster but I give the guests a chance. I absolutely hated the idea of Kratos being in the game but I tried him and didn't really mind him. Not the best character out there but not the worst.
When it comes to Freddy, I will most likely download him and try him out, but, as I said in another thread, I think it would have been way more awesome to put him in a horror franchise fighting game, which would be similar to MK with all the gore.
I think he fits in the MK world because he is a violent mofo and his story is disturbing, but, had he not been a popular horror icon and just some new character, I think it would be different. Being a horror icon as a guest in the game is just weird to me, for some reason. Yes, his character fits...but he is a character from a movie....he was not born in the MK universe...