Idk, side b seems pretty controversial among the top marths. M2k uses it a lot, and it can be an effective way of killing foxes at high percent with the regular third hit. But I'm pretty sure PP's said that it should not be incorporated into one's neutral to any significant extent, besides maybe side b -> uptilt (i forget exactly what he said but he has a QnA video on it on his YT channel if u wanna see). Even then I noticed when comparing PP against Armada in like Apex 2013 vs same matchup Apex 2015, he used a LOT less side b. It just seemed to risky because Armada could just crouch it or shield it, and there's actually quite a bit of lag after the first side b (it's a fast move, but has lots of landing lag. I think ppl get confused on that sometimes).
So basically, idk lol. Personally I've found it useful in floaty matchups to try to get a KO but its a little risky. I also like the third hit against fox at high percents as a read or used once in a while.
edit: i remembered side b can also cover spot dodge so could be good in a mixup against somebody expecting a grab =O