Apart from when you require a tiling texture on part of a model, there's really no need to go for more than one texture per model.
Multiple texture files can have an adverse effect on performance.
This is obviously engine specific, but as an example, for a game like Trainz, where hundreds of pieces of custom content are streamed into the game as you play, if many of those assets have multiple texture files, it can really effect performance.
For that particular game, I make sure that my models all use a single texture file, except where tiling is required along both axis, in which case you have to use a separate texture.
However, in a construct game, I can't see performance being a problem with multiple texture files, because you have total control over the amount that are used.
There's still no excuse for using multiple texture files though, and it's good housekeeping to get into the habit of building up your textures with all the pieces you need.
As I've said in a couple of threads now, I'd like to see embedded textures like .x DirectX models have, and seeing as assimp supports .x, it should be possible.
I didn't get a response to that question from the Devs in the latest release thread, and I haven't had time to test it myself (too busy with Android), but I'd like to think that it's going to be added to the 3D object, if it isn't already in there.