It’s quite common to find a game with a leitmotif. You tend to have a tune for villains, a tune for the heroes, etc. Technically, a leitmotif is really a part of a song that represents a mood / character / etc. However, people commonly use it to refer to common tracks or songs. For me, a leitmotif using a song tends to become annoying as the game goes on. Let me try to explain with a game that doesn’t annoy, and one that does.
The youtube video above shows a track that was never used in Xenogears. It is called the “Star of Tears.” This vocal track has elements that appear in a lot of the songs in the game. Once you “hear” this melody, you can pick it up and many tracks in the game. However, it never becomes that annoying. I think the main reason for this is the fact that the remixed versions of the song tend to have a wide range of places where it is used, and the differences are enough that it is not blatantly obvious that this melody is associated with them.
In comparison, Lunar Eternal Blue has songs by a character named Lucia. For example, this song is repeated several times in the game. Unfortunately, the non-vocal version of the tune is the “happy” tune for several areas. The cutscene below is a standard “discover a woman in the bath” type of thing, but it also shows the song appearing in the cutscene. The background theme for the area is also the same song, without the words. For me, the song became more annoying to constantly run into rather then romantic.
All in all, Leitmotif songs tend to annoy me when they’re used in excess, or when the “meaning” of the tune is diluted. Much like putting on sax music for every seduction, it feels like the game is using music with a broad brush. I suspect if I actually enjoyed the melody in Eternal Blue, I’d be much less negative about the use of it. However, since you cannot escape the main tune of the song, you can’t help but notice it again and again. Xenogears varies up the song, and uses it more as a “travelling forward” kind of songs rather than specific mood or character type uses. Lunar Eternal Blue uses the song for Lucia, happy areas, and as a song for Lucia to sing.