Can anyone explain why the mercruiser manual #9 says do not install valve seals on exhaust valves?

some believe that the little extra oil helps lubricate the guide to prevent siezing

Agreed, I don't use them at all Intake and Exhaust, better for the guides,

The exhaust has pressure that tends to push the oil away from the guide while running. They do tend to run a bit dry especially at low rpm. At most, I use umbrellas on the exhaust side, or factory O-rings on small blocks.

The intake, having vacuum, tends to pull oil in the guide just fine, and needs a little metering with either a positive seal, or an umbrella if the guide is tall enough to hide under it.

I have perfect seals on the intake valves and none on the exhaust. A small puff of smoke on start up only. keeps guides lubed

PC seals tend to leak a little more that umbrella's..

If you are using a quality, flourolastomer seal in a metal case, they are very effective at sealing. They don't "go away" over time. Now a rubber, or Teflon type positive seal will eventually loosen on the valve and let more oil by as Teflon wears and full rubber gets hardened.

The umbrellas are just as the name states...they shed the oil away from the valve allowing it to catch a mist from all around it. They are pretty much the same forever unless they crumble up and fall to pieces. I like the hard plastic ones or the plastic/rubber type. I don't like the full rubber ones...they fall apart over time.