I can't enjoy even the best game if the theme isn't working for me. I'm not fussed about whether a game is licensed or not but it has to be interesting and pleasing to my eye or I'm not going to put a coin in, and certainly not own one.
Theme also strongly influences the type of music in a game and pin music is somewhat of a lost art these days despite being incredibly important. Think back to the Bally/WMS/Gottlieb era and it was normal for games to have situational audio, catchy tunes and so on. Now we have a rash of dismal orchestral tedium on many games, and jukebox style 'this tune plays until you pick another' on the band games.
Let's look at three recent examples of where theme can make or break a game on audio:
1) Tron - the theme turns what could have been a 'decent' machine with a couple of suspect shot angles into a stellar one. Great music which changes according to the currently active mode, giving a sense of reaching goals. How awesome is it when you hit Quorra multiball? I miss my LE even now.
2) Wizard Of Oz - "we want games as complex as the 90s!". Well here one is, and while I like the theme itself when I watched a lot of videos recently I couldn't own one because that Yellow Brick Road song is so repetitive. The theme has elevated and then harmed a machine in terms of how much I would enjoy it.
3) Game Of Thrones - you could put the best game in the world out here and the music (dictated by the theme) generates a feeling of intense disinterest. I felt the same way about LOTR, boring orchestral film music. I can't even tell when the tune has changed really, I just don't give a fuck.