Why ball valves on drain line and/or return line?

Precision isn't really an issue with a return pump and gates really are not designed to throttle with either, you place unequal stress and wear on the sliding gate and scour the seat out.

 

I find them far easier to use.

 

Suggested flow through the sump is up to 10X the sump's volume. You return pump should be rated about that flow at whatever head your return nozzle has. This is normally 4 to 5 feet.

 

A good plan.

A bad plan.

You don't; you're exactly right. Not only is it a (bad) restriction, but it's a good potential clog point. And a drain clog never ends well.
People (wrongfully) do it to slow down the drain flow and therefore silence the overflow gurgle. But this is really using a sludge hammer on a pushpin; there are much better ways to go about making an overflow quiet.

Now this is a different story. Throttling back a pump often makes the pump run more efficiently (there's usually a sweet spot for this). It also lets the user "tune" the pump flow to what best suits the tank and overflow. It's easier to "overbuy" on the pump size and throttle it back some if needed, rather than buy too small of a pump and "magically" make it pump more.

Will lower the GPH, yes, but it won't burn out the pump (unless it's throttled on the suction side - BAD idea). Now, if you had it really throttled back to the point the pump could no longer get sufficient cooling, then you might harm it. But that would be with the flow nearly shut off at that point. Also, lowering the GPH makes the pump consume less power.

 

Best advice all day right here:

 

We need a reason for valves?

There are only two mistakes one can make along the road to truth; not going all the way, and not starting.
Buddha

 

I really appreciate the help, everyone! I'm definitely enlightened. It looks like I'll have a valve on my line to the refugium that T's off of the drain line, and will have a valve on the return line somewhere.

Mucho appreciated Karma to all that have helped. Especially Evolved. He nailed it all in one go.

 

And I knew there was a "10X something volume" rule for return pumps, but I thought it was the display tank, not the sump. Yikes. Will definitely do that right this next build.