My rule of thumb is that if the ride is two seats across, it's a lot less valuable. Three or more, and it's usually worth it. (Most guests are in pairs / even numbers.)
I rarely SR Incredicoaster. It's such a convoluted system, and takes forever - I'd rather just use an LL or wait until the line is short (often late in the day).
I'm surprised Soarin' even has SR. If a CM is perfectly efficient (which, to be fair, is extremely difficult), there should never be an extra seat for a single-rider. Better to just pull the first available 1 or 3 out of the line. (I probably wouldn't SR this anyway - too high a likelihood of ending up with a bad seat. I won't even respond if they ask for a single-rider in line - that dead last seat in the back left corner is awful.)
WS is really good with SR. There's a weird plus-minus, which is that you end up spending most of the time waiting in the outdoor queue, which makes it seem way worse than it actually is. The SR hallway is only accessible via the left pre-show room (every other pre-show), and they only put a specific number through each time (I think ten), so it feels like you're spending forever standing around with nothing happening. But, once you finally get to the pre-show, it's usually really quick after that.
I tried the newer Space Mountain SR the day before rehab, and it seemed quick, but mainly because nobody knew it was there. And it was hit-and-miss that weekend - it wasn't open the previous Friday night. The old version of the SR line could be really hit-and-miss - there were times that the line spilled way down the hallway (which I think was why they got rid of it, since it interfered with ADA access).
I'm one of the random folks that actually likes being Engineer on MF:SR. (It's fun to see how fast I can "fix" everything. It was especially fun the time we had a three-year-old pilot.) The trick there is to do the u-turn to the second SR staircase that most folks seem to miss - usually an even shorter wait.
Side note: I liked the Pirates SR test that they did last year - I was a little disappointed that they didn't stick with it.
yeah it all comes down to the seat configurations.
4 person cars (Goofy, WS): Any group of 3, 5 or 7, they're going to need a SR. Should moves very fast.
6 person cars (MF, RSR, Matterhorn): Any group of 3 or group of 5 that comes through main line, they're going to need a SR, unless they can find 2 groups of 3 to stick together. So these move fast. These have a lot of ride vehicles too, so they move quick.
8 person car (GRR): the ride vehicles don't turn over as fast, but they usually can't puzzle together groups of 8, so you get one SR every 2 or 3 boats
Space probably has a lot of variety as it depends if CM treats it as a 6 or 12 person vehicle (i.e. are you willing to split a group of 5 across the front vs back car).
When Indy had SR back in the day, it moved slow because of the 12 person vehicles.
Incredicoaster is 24 person, and inevitably any odd number has to be placed with a stranger regardless, so SR spots don't pop up that often. Cars move fast though.
Soarin I think they treat it as 9 separate vehicles of 12 people (each row/column treated separately) as they want the whole group to be in a single section. But because they have so much time to organize and move people around, they can optimize it a lot more.
Guardians would be even worse if it ever had SR - 21 person vehicles, lots of time to get shuffle main line guests around!
And yeah, pirates basically never has full boats, so SR would be great there. I think they just feel the line moves too quickly to warrant the organizational hassle. Small world is same boat.