I used to be Single-player exclusively, but that eventually got boring.
I've been "playing" mostly in multiplayer for the last few months, but whenever I want to experiment with a build, I drop back into singleplayer, as it's MUCH more stable than even the best-run servers.
(EX: on a server if my internet or power craps out, I'm hosed [or, heck, if the Server's internet/power takes a dive]. in single-player, I only have to worry about power, and I normally see that one coming)
Part of why Multiplayer is "better" is that you have someone to talk with, to show-off your builds with, to go shooting up Pirates with.
And, naturally, to sometimes wage War, limited or total, against other people.
If I weren't so terrible at it, I'd even do some collaborative builds. (I just Don't build well with others. Build off of/around their work? Sure! But not with for some reason.)