My understanding is that most domestic unvented heating systems use the
boiler's pump to circulate water and on/off valves to divert water to the
hot water cylinder or heating zones.

Under what circumstances should the zone valves be replaced by zone pumps
with a mixing header between the pumps and the boiler? What's the function
of the mixing header, and do you always need it if you have any heating
circuit pumps or DHW pumps external to the boiler?

Does a zone pump arrangement confer any control advantages over using a
boiler pump + zone valves?

(I'm currently considering buying a 38kW boiler which offers a vertical low
velocity header as an accessory:
http://www.mhsboilers.com/boilers/premix/strata1.htm)