Throttle a pump harmful?

I've read here and on Marc's tank website that you shouldn't throttle
centrifugal pumps with a ball valve but that you should divert some of
the flow back to the intake side.

I hope I'm not being obtuse when I ask, "Why?"

Speaking specifically about Mag pumps, the manufacturer doesn't warn
about or otherwise recommend any minimum flow rates through the pump,
and the backpressure from a constriction in the line should be
identical to the backpressure from a higher head application.

At least that's what my math comes out to. I don't think the pump
can tell if the reason it's not pumping as much is because there's
a 5' head or an almost closed gate valve in the line.

If I'm wrong about this, could someone please take the time to
explain how the two cases (partially closed valve vs. increased
head) are different from the pump's perspective?

Or, alternatively, something else (not necessarily pump damage) may be
the reason to keep the flow through rate at a maximum. Perhaps
something like plankton mortality rates (just stabbing in the dark
here) that get much worse if the backpressure goes beyond a certain
point...

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller