Two days ago, had an interesting encounter with a man 20 years younger than me, in the disused guard room of a missile area on an RAF base, being used as registration for an orienteering race. He'd read an account I'd written about completing a mountain marathon (MM), then traced me through East Anglian orienteering results and photo on an adventure racing website. He's been using a pump for 3 months, and having done a MM on MDI, was seeking advice about doing another MM with a pump. However, when minimising weight is vital, as you're carrying tent, stove, food, sleeping bag, any clothing not being worn etc. Non essentials for 1 night camping aren't taken eg no toiletries, not even a cut down toothbrush; sheets of bubble wrap instead of a sleeping mat; no spare shoes, so replace wet socks with dry spares at overnight camp, then plastic bags on feet, then back into wet shoes etc, having to carry back up pens might mean he continues to use MDI for MMs. For shorter orienteering races, there's possibly an advantage, although I didn't have to carry my pen round the course, while he had to carry his pump; we each had a plastic bag of jelly babies in our pocket for the run.
For me, lots of time spent kayak touring, staffing adventure race checkpoints or transitions, sometimes miles from road and for days and nights on end , etc, make a pump less suitable than MDI - "if it ain't broke, don't fix it"
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