There are nothing more stunning in card magic than a card to change its identity. I think I'm paraphrasing Vernon, but some one can correct me if I am wrong.
With that being stated, I don't bother learning all changes, but having all those changes out there are kind of like having a buffet. You can pick and choose depending on your skill level, style, and etc.
As everyone stated before each change has its pros and cons. I use only about 4-5 in my performances, and I usually go for the angle friendly ones. Others go for more visual ones, it comes down to your preference and how many people you are performing for (angle).
Caladan21ph, if you are referring to Erdnase color change, I have few subtleties I've learned that may help. If you are interested, feel free to PM me.