The derez is part of the subroutine — if it is broken, the derez never happens.
Cell Portal is to be the innermost ICE on a heavily guarded server (likely remote server 1). The runner breaks all the way through only to hit Cell Portal and get booted all the way out again. It plays a lot like Toll Booth, as a way to waste runner credits. Even when they break the subroutine, breaking a STR 7 Code Gate isn’t generally cheap. But I don’t like it. It hinges on having other ICE in front of it, which makes it a bad draw early. But then you need it as the innermost ICE, so you want to draw it early. No win!
It’s also good to make the opponent go back through ICE like Neural Katana and possibly take lethal net damage.
Cell Portal is junk.
It just got a lot better with the new Jinteki card that re-arranges ICE on a server. Any server with 4-5 ICE on it will be tough as hell to get through now.