Well I do both and I think the streets can help your trail running both mentally and physically. You have to be alert on the streets just like you do on the trails and the hard surfaces will toughen your body.
Plus running the streets is like trail running in that it is not a steady run. You have to stop and go, avoid obstacles, chose the right path and some times make difficult moves on the fly.
So next time you can't be on your beloved trails, do some urban exploring!