They're not really the only. If you subscribe to OnLive's How About You Pay Us Every Month package, you get access to Homefront's multiplayer only. In theory it would essentially mean Homefront's multiplayer won't be wanting for players for a long time. (In practice I don't own it so I don't know how well its multiplayer is doing.)
There's a slightly poisonous idea amongst devs/publishers that people will not buy games unless they have single player. I guess it's another part of Modern Warfare's heritage; that is a game that I can see 'come for the single player, still for the multiplayer' actually being true. But I only played Bad Company 2's single player for two levels, because that's how long it took me to realize it sucked, and I resent every single bit of space it took on my hard drive, as an unwanted tumor stuck to the game I really wanted to play.
Most multiplayer-only games are either free with microtransactions or $15 dollar affairs, and if companies aren't making back their investiments with $60 AAA games their reluctance to publish things that are even cheaper is understandable. I doubt a $15 dollar game would cost less money to produce than and a $60 title and it's not certain it would sell more.
I would enjoy such games coming out more often, but I doubt I'd find one who would have me abandon TF2.