After being a Spotify Member for over a year I recently had deal with the hot topic, vanishing offline tracks. This happened weekly. I eventually decided to close my account and move to a new 3 month trial, all was going well, then wollop I hit maximum downloads and my library of albums (not all downloads) also have a limit. Then the old problem pops up just before a long road trip offline tracks disappear. During my 2nd attempt to stay loyal to Spotify I have also run a Google Play Music Trial. None of the above problems have been an issue, so sorry Spotify I am not going to pay for the service that isn't delivering.
I will however return when your download limit, library limit are fixed to something more realistic and technicians admit the offline listening side of the product has a bug in the software.