The spiral saw is a specialized tool--it's basically a router with a 1/8 inch collet. Trouble is that between the wobbly base and lack of any kind of precise depth adjustment it's a lousy router. If you need to cut drywall or plaster that's on the wall it's your tool, no question, because you can set it to go through the plaster without chewing up the studs or grabbing any wiring that's in the wall. But that's it's only real strength.
Since it does have a 1/8 collet it will take Dremel bits, so if you ever need Moby Dremel it will do the job but so will a ten buck die grinder from Harbor Fright (if you have a compressor).
For the tasks described it wouldn't be the best option IMO. With the standard bits it does OK on plaster and drywall but it's horribly slow on wood thicker than 1/8" or so and doesn't give a smooth cut either. Most of the rotary saws will take a 1/4" shank router bit and have enough power to swing one if it's not too big-with one of those it will zip right through wood but the base is really too narrow to get a good vertical cut and it tends to march to its own drummer unless you have good jigs constraining its movement.
For what was described, I'd likely use either a jigsaw or a Fein Multimaster depending on what was behind it and how close it was to obstacles. The Multimaster is near its depth limit at 1-1/8 (blade geometry limitation, not power) but it will do it--if I had to get only one tool for what was described, and not having eyeballed the situation, the Multimaster would likely be it.
I've not used the Rockwell that nailshooter mentioned--if it had been out when I got my Multimaster I would likely have given it a good hard look though. The oscillating tools are the opposite end of the utility scale from the rotary tools--they have so many tricks they can do that you wonder how you got along without them.
A consideration though--the blades for all the oscillating tools are fairly expensive--if one is using it heavily then it's going to be cheaper to get a jigsaw for the grunt work than it is to keep using up 20-40 buck blades, and reserve the oscillating tool for the jobs where its unique talents are really needed.