Because I got tired of switching the blades every time I wanted to do something different, I've got no fewer than four knives, three with #11 blades, but each knife has a specific purpose. First, I have one that I use for what I call "general cutting." This is removing parts from sprue, scraping off excess plastic, cutting sheet styrene, stuff like that. If I want the extra sharpness of a new blade, I'll go to the knife that has the brand new #11 blade. (When this starts wearing out, I'll switch it to the "general cutting" knife and the old blade from that knife gets discarded.) Finally, a #11 blade that was dulled by scraping it on the cement is in the third knife; this knife is the putty knife. I dipped the handles of two of the knives in different colored paints for identification.
The fourth knife has a #10 blade in it. I don't use it real often but it's handy if I have to cut at a certain angle; it's especially good for cutting strip styrene. The handle on this knife was also dipped in paint. For heavier duty cutting, I have a handle with the #2 blade and even a couple of saws. Those are things I almost never use, but when I want them...