I'll admit I'm a bit disappointed, but at the same time just getting to see Tennant's
Doc again sounds pretty damn cool.
And I don't buy that it's because Moffat has no respect for the past. He probably just thought it would be a lot more fun to watch these two Doctors go on an adventure together, than to have another special with 4 or 5 Doctors talking and bickering endlessly for an hour (which is usually what tends to happen in these things).
And as others have said, there's many more ways to pay respect to the history of the show than by just trotting out the original actors. In fact that's probably the least
original thing you could do.