It's just that it's going to be tempting and so easy for the DM to favour his own character in some way, be it through putting in loot that is perfect for what his character could use, fudging the dice rolls to save his own character, etc. Again, if you can do it then awesome, but for a lot of DMs it's super hard to treat their characters the same way as the player characters.

I love the BoVD :D The alternate curses that they have in there are delicious! Also, one of my spell domains is from there, for some REAL dark juju.