And the curvy texture on the pieces we would get would be too hard to put together, because it would be hard to align edges with rounded edges.
That isn't to say there couldn't be Covenant themed forge pieces as part of the palette for a non-Covenant themed forge space.
I haven't done much forging since Halo 3. If there would be any type of map I'd like to forge on it would be one that resembles Foundry! That or another Forge World. I guess it's time to actually see what the current Halo 4 maps look like.