The handheld-rubber duck approach has lots going for it, and in my opinion we should avoid “talking it down”.
Activators embarking on a strenuous multi-summit round in G/LD will want to minimise pack weight and volume, and so a handheld + RD is perfect, especially with lots of like-minded SOTA people taking part in the event and in those valleys, on those hiking routes or on other summits. HH+RD is therefore pretty-well perfect, and furthermore is the most considerate low-impact option on popular/busy summits on a summer weekend.
I am so impressed with my Yaesu FT70D, as you may have read. I would be highly confident of doing much better with it than just valley bottom contacts - even with just the rubber duck.
I’m sure plenty of participants in this event will utilise beam antennas and/or use HF / 2m SSB etc but anyone that chooses to take part HH+RD style would have my full support as it is the ideal approach in so many ways.