For Team Deathmatch, that'd mean one team could easily barricade themselves in one end of the map.
For Deathmatch you'd have to limit the map to a small segment so people would not spread out too far.
You ever play Left 4 Dead? You got these big long maps to play on in normal mode. Point A to point B. Now there's another game type called Survival where you have to, well, survive hordes of zombies one after another. But instead of having one long map to play that on, Valve only used parts of the map, modifying them so you couldn't go any further and so there really was no place you could hold up.
See, all SD would have to do is pick sizable sections of a map and edit it a bit so you were contained in that one part. Lock a door here and there, block off a hallway, etc. Set up spawn points, name the map, then you're done. It's really not that hard.
Of course another way to keep people from holding up on one end of the map is to put something in the center of the map that could help whatever team gets it or something like that. Bad Company 2 usually puts a tank in the middle of the map. This is also why in some online FPS games they put the biggest gun somewhere in the middle. People are always fighting over it. It gives them a reason to be out in the open and fighting.
Now why you say a game mode like Conquest wouldn't work confuses me. The Conquest I'm familiar with (from BC2) you got three points you're fighting for control over and whoever holds the most points the longest wins. Now you could hold up on one end of the map at one point for the entire game but that would just cause you to lose the game.