What does anyone think of a PPL holder that flies only during vacations/holidays. Assuming he is able to put 20-40 hours a year in one or two spells of two weeks annually with intervals of at least six months, will he still be a proficient and safe flyer?
That's a maximum for 4 weeks available to fly a year. In this country? No chance. It'll only actually be flyable for two of those weeks, and getting 20 hours in 2 weeks is very hard, and knackering. Anyway, what sort of flying would you pack into two weeks? A very long tour? VFR? His/her chance of getting back in time are slim.
In economic terms is the training expenditure justified if the pilot will fly infrequently albeit in short intense bursts?
Depends on the pleasure s/he gets out of those bursts. 20 hours of local bimbles packed into a couple of weeks wouldn't do it for me.
For safety ,is frequency of flight more important than absolute number of hours logged?
Yes. Very yes.
If PPL flying skills decay due to disuse, what is the best way of mitigating this?
I'm not aware of any. Sims won't hack it IMHO
What form of aviation has the least 'decay of skills' due inactivity?
Thinking about it, dreaming about it. Can't think of any others.