It can be. First off, its worth looking at the tier list moisterrific set up which I think mentions which cards are worth evolving. That's for optimal play though.
I personally evolved my Bassline Big Band and he's currently my strongest silver card. It's definitely a fast way for more power as silver is more powerful but the skill tree is still as cheap as it was when the card was bronze, so you get skills faster (my Bassline is also my only character with all five skill trees completely filled).
So if you need a little boost through the story and some help with dailies, my recommendation is to go for it. It definitely helped me in the short run (especially with all the energy cooldowns skilled into which meant I could use my (then) most powerful card much more often).
Also, no, you don't lose unlocks in the skill trees, though it will set the card's level back to 1.
