actually, at +2 outspeeding rotom is splendid, since it's pretty common for rotom to try a revenge burn. but outside that i don't see any use (better chance of outrunning azumarill? generally i think you do that with any investment at all...)
I just ran the calcs on 252/252+ Rotom-W, a Jolly (216 EVs to outspeed, since everything else between base 86 and base 91 runs speed so you won't be beating them) +2 M-Mawile will OHKO with Play Rough as long as you have 92 or more Atk EVs. That doesn't seem right, but I'll take the calculator's word for it. You're literally going Jolly just to beat Rotom-W as they try to revenge though, everything else that could bother M-Mawile between 81 and 86 base speed will be running enough EVs to outspeed you regardless. It can work, but greatly reduces M-Mawile's potency as a wallbreaker so whatever your team needs more I suppose.