I agree, quality is important over quantity, but I guess we all have our "filler" titles for whatever reasons (usually because they were too cheap to pass up).
Personal "guilty pleasures" are also a factor which drives collectors to buy what they do. For example, I love collecting the Cooking Mama series of games, despite ridicule from fellow collectors, but I don't care because I actually like playing the games and like collecting them just as much. Whilst they may not hold any real value to them from a collector's point of view, they hold value for me because these are games I enjoyed playing. Hmm, perhaps I shouldn't have admitted all of this. Lol!
I own games I bought because I enjoyed them. I own games I bought because they were cheap and in good condition. I also own games I bought purely because they were rare and worth a lot of money, despite the fact that I would never have given them a second look otherwise. It's a mixture of these three things that makes every collector's collection unique
As you guys may already know, I am obsessive about condition, and I pride myself on the condition of my collection as well as the quality of the titles I own. If I own any sports titles, for example, it's usually for a good reason, and each has a a story behind why I have it - Such as FIFA 2001 because it's an infamous "scratch & sniff" disc, or all of those crappy Dreamcast sports titles because I am aiming for a complete set of Dreamcast games.