Signs of worn and leaking swirl valves are if the rod that sits on top and links all the valves together falls off because the valves start to lift in the manifold, an oily build up around the valves shows they are leaking and if the rod is off you should be able to turn each valve except the one that is connected underneath to the actuator, on the 4 cylinder engine it is No 3 but not sure about the 5 cyl engine. If you cant turn them then the ports will be heavily coked up.
Advantages of removing the valves are its a lot cheaper than a new manifold, £30 plus a few hours v about £350 for a 2.4 manifold. and you wont have the same problem again, though the new manifolds have a better design valve bush setup.
The manifold then probably flows a bit better so potentially a bit more power.
Yes the manifold has to come off to fit the blanking plugs, a big job.