I have the M&P Shield and YES I have a thumb safety. Reason.......when I IWB carry, I don't want to risk an AD/ND. Not that I am careless, but freak things are know to happen.
I do realize if I had to engage the pistol in case of an emergent event, the small safety lever could present some delay, but IMHO, it worth the slight delay for the peace of mind that the safety is ON!