You don't have enough barrel length in a snubby to get use out of +P
Sorry, but I don't think that makes sense. I've read too many chronograph readings and ballistic tables to buy it. The old +P FBI load, to name just one, has a long track record of effective use in both two-inch and four-inch guns.
It may be true that the advantage is less in .357 Magnum. I don't know. I think .357 in a snubby is a dubious idea for several reasons, but in .38 Special I like it a lot.
As for strength, my dashless 640 is etched inside the frame "Tested For +P+". I only carry +P, and practice with standard-pressure FMJ, but I've read a lot of well-informed (read "expert") opinion that makes me think occasional use of +P is not going to blow up any reasonably-cared-for steel J-frame.