We were on the Mariner a few weeks ago and yes, you can get photo's taken for all kinds of things. (i.e. when you embark,formal nights,gangway photos when you get on/off the ship at different ports of call - and even on some excursions like waverunning at Labadee ).
I say go ahead and get your pics taken
Even the formal night pics that you have taken have NO obligation to buy and no sitting fees so what do you have to loose? Get them taken and if one or two come out really good then buy it and if not then don't.
The great part is you don't have to buy them if they don't come out good.
We bought a few simply because we liked the background - loved the Titanic background on one of the formal nights and the embarkment photo is a really nice keepsake.
But again, if you don't like then you don't have to buy.
HTH's. Have a great trip!