I default rate mine as "2" as well. My "system" for the rating stars is:
5. The best songs in my Library by my favorite bands. These are rare. You can't get a 5-star rating unless I've had the album for months or years and still love the track.
4. The best songs that don't quite rise to the level of a 5-star, or maybe I haven't had it long enough to decide yet (because that initial blush of greatness can fade with familiarity).
3. Above Average track.
2. Average, regular, run-of-the-mill track.
1. Bad track that I don't want to delete (because I want to keep a "full album" or something, or for humor value), but which is excluded from essentially all of my auto-generating lists.
There's no particular reason to default it to 2 instead of leaving it unrated, but since I have a wide swath of auto-playlists that work based on rating, the Expression logic is a tiny bit simpler if I don't have to account for unrated tracks.
Simpler expressions are always better for me. Not because I can't figure it out once, but because I still have smartlists in my library that I designed literally 6 years ago. In 6 years, will you still remember how that smartlist expression works (or why) intuitively?
To me, that is a very important design consideration when I'm structuring my library.