I doubt it's intentional. I was played a game with frogboy and was trying to get him to help push a portal early, I think at war rank 4 or so, and told him that it was worth it because it would give the whole team gold and exp. His reply was that he knew how it worked because he designed the mechanic, and in spite of that he still didn't seem to value the portal all that much. So to me it seems unlikely that the intent was to make the whole game into a flag lock struggle before rank 10, but who knows?
Maybe Frogboy himself can shed some light on it. Basically right now capping that port shuts off creep waves and lets your creeps demolish the front of their base while they fight for the port, it gives you a gold mine, and it gives the whole team some gold and exp as the waves come out.
You can lock one and then port to help out ina 3v3 across the map and still get exp off that lane while you're away. Cow is right that it's specific to Cataract, but I would say it's a problem specific to cat rather than a feature. Regardless if people would just stop playing cat over and over again it wouldn't be such a problem.