I did take people's advice and start out with dev textures, but they were obviousl ugly and I just added real textures.
The point of not adding "real" textures in early alpha/beta is because you're probably going to be making a lot of layout changes, and as such, if you detail the map, you'll lose a lot of time and effort.
There's no shame in having your map full of entirely gray, red, blue and orange blocks if it plays well. Then, you should start detailing.
Of course, this is coming from me, who details a map before even deciding what game mode I want it to be.