Wait, I have a few videos with copyrighted music but they never got muted, just the name of the music appears on the side, does those videos will be deleted or something with partnership?
If they will be deleted then I won't doing partnership
This is my understanding of partnership, if you apply to be a partner then they may monetize
all your videos. So if you have any videos with copyrighted material at all, they will probably reject you. FreddieW was rejected a couple times because of his guitar hero video I believe. Nothing will be forcibly removed, but you won't get to be a partner, and if a company sees that their material is in your video then they may either leave it to help advertise themselves, or they could order youtube to take it down.
The only real pluses to being a partner I think is that you get money, (and if you have a sort of small audience, it probably isn't worth it,) you get to choose video thumbnails, and you get a few more small graphical tools for your youtube channel. Other than that, not much. I don't think it's really worth it. Since I'm using BL my videos will never have that many views, and being unable to use copyrighted material will make video making much more difficult.