I had two 403(b) plans from the same employer. One was a traditional (tax-deferred) plan, the other a Roth 403(b) that became available much later, I think in 2008. Once it became available, I stopped all contributions to the traditional in favor of the Roth.
After I left, at some point I converted the traditional in its entirety to a traditional IRA. I left the Roth 403(b) in place. I'd been doing some shifting of investments this year among plans (I also have a 457 plan) and realized that the Roth 403(b) offered some share classes (institutional) that were not available in the traditional IRA (both are with Fidelity). The expenses can be quite a bit lower for the institutional ("K class") funds, so I'll stay with that.
It happens that the Fidelity fund choices available to us were quite a few, so the common problem of lousy investment selections in an employer plan wasn't an issue for me.
I also keep the 457 as it was, and a big reason for that is that it offers a stable value fund, which is not available to me elsewhere.