@Feryll - Not removing the old paint from the porch will save you time and work, but I think removing the paint is worth the effort. When the paint is already chipping and worn you can remove most of it with a paint scraper and elbow grease. If you have a power sander then that's even better.
Either way, getting as much of the old paint off as possible will make the porch look better. The new primer and paint will go on smother and it will better protect the wood. The last thing you want to happen is for the new paint to start coming off because it's not sticking, and then you have to do the job over again in a few months or a year.