Spotify uses a SQLite database for playlists. It's called mercury.db and the problem is that they are issuing VACUUM commands to repack the database very often, possibly after every write. VACUUM works by writing the database to a new file and swapping it with the existing file so it will create an enormous amount of life time writes especially if the database is huge which for a lot of customers it is.
I believe they've since fixed the issue in the latest build of Spotify. I would like to point out though, MLC based SSD's like the Samsung 840 Pro are very resilient. In independent testing the 256GB 840 Pro from Samsung was able to handle 2.4 PB (2,400 TB) of writes over I believe a year during Techradars testing. And that is the smaller 840 Pro, they go to 1TB and the more NAND you have the higher the writes you can sustain.
Samsung themselves cover the warranty on their newer 850 Pro's at over 550 TB of writes a year so if you're under that amount and your SSD dies within 10 years they will replace it for free.
Of course I'm not suggesting this makes it okay, what Spotify did was incredibly stupid, I'm just illustrating that for anyone worried about it you're probably fine as this bug has been caught quickly. I personally do not use Spotify.