The benefit is that once you have glassed these into the boat they will last a lifetime and never need replacing. Even if you replace the seacocks ten+ years down the road the backing plate can be re-used.

When should I replace my hulls?

You can check the thru hulls first when scraping the paint from the metal and if it looks golden and not pink, it is in good shape. If the metal is pink, the thru hull fitting should be replaced immediately, since the metal has no strength and is ready to fail.

Can you replace a seacock in the water?

It can be done, just have a plan B. I would advise the use of two pipe wrenches one to hold the thru-hull from moving and one to turn the valve off. This will bugger up the thru-hull threads but it's better than breaking the sealant bond.