No reason. The two versions can exist side by side on your computer. It might be nice to have 2014.5 "just in case".

I confirm, they can co-exist without problem.

Not only can the two exist, but you may find it useful to be able to reference your earlier installation to see where certain files go. (That helped me recall where various files for added Garritan packages need to be.)

I have Finale 2012 installed on my Mac. So, it's better to leave it installed together with v. 25, or I can uninstall 2012 without any problems?

The choice is yours : either you leave 2012 installed, or you remove it. There are no interferences, 25 and 2012 can coexist. And no problem if you uninstall 2012.

However, note that if you keep 2012, it works on recent MacOs only if Human Playback is desactivated.