True, all the detail you put into a map will be seen and gone in a couple of seconds, but without it, the map isn't anything special, you need the detail, because people do take it in as they go through an area, even if it is only for a second.
I prefer SP slightly, as you can have a story to it, and really immerse the player.
You have to guide the player, and try to guess what he/she will do at each area, so as to create the most effective atmosphere.
I agree with several people here on mp mapping, most maps never get played, and you can't make the map too big a file etc. And when you have made it, what can you really do with it? You can walk around it by yourself, you might get maybe your friend to play for 5 mins, but that is it.
WIth SP you can just play it by yourself, and have plenty of custom content, and SP maps are far more likely to be played by someone, since they don't need another 7 people to actually see how the map plays.
/opinion.