So maybe it was thinly veiled attempt to collect/update marketing data after all. I.e. it wasn't for safety while on premise at a hotel. Either way, whenever I am asked, I just provide the number. Yes Marriott should already have my phone number from my profile. In practice that may not be flowing into the PMS system at each property. I don't know and it isn't worth fighting over for me. YMMV....

--Jon