The wear limit is 1/4 tooth on the camshaft sprockets. Chain, camshaft sprockets and crank are to be changed as a set. Some guys like to change just the chain. Not the smartest thing to do if the sprocket is worn. I personally would not recommend putting a new chain on worn sprockets. You are going to end up accelerating the wear on the new chain and already worn sprockets by doing this.
I my experience I do a few top ends with the same chain/sprocket set. Then when I split the cases I change everything as a set. It's how I have always done it and is what the service manual recommends. In reality you should not have to change any of those items during a routine service on the ST.
I don't HAVE to change it. I WANT to change it, and if the bike's apart anyway, for another $22, i have a new chain. With Hy-vo chains, it's the chains that wear, not the sprockets (and we've been through this before). When i was an engineer with GM Hydramatic, the 325, 125, 440 FWD transmissions (this was 36 years ago) all used Hy-vo chains. Chains took 97% of the wear, sprockets 3% (if that).
Hy-vo chains are NOT roller chains. What is true for roller chains is NOT true for Hy-vo chains. You really don't seem to understand that. Roller chain? Yes, change sprockets with the chain. Hy-vo chains? Completely different animal.
I probably change the timing belt on my 1992 volvo 740 too frequently, but after 27 years of ownership, i can do it pretty quickly. If i change the accessory belts, i throw a new timing belt in it, too. It takes an extra hour, and i know that it won't be an issue. 27 years and 300K miles later, the old girl is going strong. I can buy a continental timing belt for $8 from rock auto, so why not?