I'm getting ready to brew a robust porter in the next week or so, and I'm planning on 2 hop additions (UK Phoenix at 60 and Kent Goldings at 15). I brewed a very similar one last fall, but that one was dry-hopped. I decided the dry-hopping didn't add much, so I dropped it this time around.
I am keeping the 2nd hop addition instead of just doing a bittering charge for two reasons. First, I think it adds a bit of depth to the bittering. And, I don't want to change too many things each time I brew a batch. This time around, I'm already dropping the dry-hop, changing the grain bill slightly, and adding vanilla beans to secondary.
I think whether you decide to do a single bittering charge or add some flavor/aroma additions is mostly a matter of preference. But, in my opinion, a flavor/aroma addition would have to be pretty big in order to cut through the dark malt character of a porter.