I just wrote this on Doomworld and decided it would make a good post here as well.
Re: Single Player: Yes. Cooperative: Yes. Deathmatch: Yes.
Solo/coop and DM are two very different games that just happen to have the same graphics and rendering engine. They have two very different sets of requirements for a good map. Trying to cater to both modes at once will result in their cancelling each other out, and a lacklustre map. For example:
SP maps have a direction, a upward slope. You start at the beginning and have to work your way to the exit, which is not immediately accessible (monsters, locked doors, etc. are in the way.) Conversely DM maps are undirected and flat. You can be spawned and respawned anywhere and (almost) all places must be equally accessible by all players from the start.
DM maps are small, as the player has typically less than 8 opponents as opposed to dozens or hundreds (or thousands...) and the map needs to be small enough so that the players happen upon each other constantly. Duel maps made for 2 players are an extreme subset. Conversely SP maps are much larger and those that are small enough to work as DM maps are going to be finishable very quickly (not that's particularly a bad thing, but sometimes you want to offer something more substantial.) At the very least you have to trap the DM players in one part of the map, in which case you might as well just stop faffing around and make a separate DM version.