Simple. I love knives and bladed weapons...swords, spears, halberds, etc. Each tool has its own unique characteristics no matter how similar the seem to be. The more experience that one has with the myriad varieties available, the better prepared one will be when using and defending against them. It's kind of like studying martial arts and only practicing against your own systems' attacks...no good. Most likely, you'll encounter someone that approaches fighting differently than you are used to and you better know how to deal with it (if that's what you're into). Even if you don't care for a certain style (or knife), it is a good idea to know about them.
Anyway, I try to stay away from impulse buys, but sometimes find myself buying based on a perceived "cool factor".
Other than that, once you have been afflicted by BAD (blade acquisition disorder)it is nearly impossible to stop.
This is the basis for my knife purchases.
Happy Holiblades!
FM.