Not op but I'll chime in. My team uses npm for our build tools and bower for our frontend libraries (moment, chats, lodash, etc). Our re-write only uses npm but at the time (and possibly even now) there were frontend libraries that only provide bower packages and bower seemed like the right tool for the job anyways.

It doesn't sound ideal having two tools that do pretty much the same thing, you are just adding a potential extra point of failure. Migrating from bower to npm is incredibly easy.

I remember wondering this same thing myself when Bower was the the new hotness.

"Don't I already have a tool that does exactly this?"

How would you migrate libraries that are only on bower, not npm?

How many of those are a) left and b) still maintained?

If there are still any in 2016 it's a good sign they are not begin actively maintained