It is situational.
We ran SM for several reasons, over and over, until just recently when our group started to hit the 40s. (It is an alt group that we created solely to play together certain nights of the week)
We have a paladin and a warrior in group and they both were able to pick up several pieces for use. As neither of them has unlimited funds to just go buy all the armor they want off the AH, it was viable for them to collect the SM armor. They won't be immediately replacing them with plate and until then they will have some nice mail to fill the spots.
We had a lot of fun running our group through SM, and the loot was an obvious plus. I'm not sure if it would be good for you to "go out of your way" to do the instance over and over just to get the full set, but if you enjoy doing the instance then why not? At least then you would not be frustrating yourself getting replacements, even when you switch to plate.
At least post-40 your quests will start to offer plate rewards.