if you learned/trained with a safety like a 1911, then yes get it. if you haven't and started with say glocks, then no.
i disagree with other's here. having too many safeties besides your mind and trigger finger is not a bad thing. it's all in how you train, and what you train with. i do find those who started on glocks tend to forget to check for safeties when trying to shoot a 1911 or other safety type design. which could cause an issue in a self defense situation, if not carrying a glock, etc. whereas those who started on a hi-power, 1911, etc tend to not have issues shooting a glock type quickly and safetly as well as a safety pistol. ymmv of course, everyone is diffferent.
although having said that the ultimate safety is not pulling the trigger, and hence even when training with a safety, never use it as a failsafe, period. it's mechanical and could fail or you could switch to a glock and have already set yourself up for a bad habit.
i don't own an m&p yet, but i played with a couple at my local store and the safety isn't hard and firm/secure feeling like my hi-power or 1911 or cz75 and 85 combat, it was effortless....so much so i thought why even have it as it could get clicked off without even realizing it. having said that if i buy an m&p i will get one with the thumbsafety, cause i like safeties and have trained with them.