Of course you will lose the possibility to play your beloved vanilla, though .
They sell the MM's quite cheap, and from time to time they put them on sale, which means that they're dirt cheap at that moment. I haven't ever had disks for MM7 & 8, so I purchased them there, and they happened to be on sale. I paid something like 2.5 Euro total for them. I played through their MM7 without any problems. (Though I added Grayface's patch and played under Wine, which often works much better than playing the old Windows games under Windows!)
GOG also makes it the priority that the games suffer from no DRM, additional "clients" etc., and they do a great job. (And they have to, because some of the customers can scream very loudly whenever they see anything that looks like DRM, even if it's not. I would even say that some are a pretty nasty lot. You can routinely read comments like "YOU MUST CONNECT TO THE GAME SERVERS IN ORDER TO PLAY LOCALLY! THAT'S DRM! I WILL NEVER BUY FROM YOU AGAIN!!!!!", while the others try to calm them down by pointing out that (e. g.) if they stop screaming and click the button "Play Locally", they will be able to play locally.)