I don't question that - I am one such person who travels quite often from another part of Cornwall. Neither is there any doubt that the Park and Ride is busy, especially at those times. A 4 car train is needed, if not more, during the summer and I can remember 5 car set ups in the days of the Modernisation Plan dmus and railcars. However, a 4 car train from St Ives will lose much of its load at Lelant Saltings and at St Erth some people will want to go on down to Penzance. However, a 2 car train coming in from Penzance may well be already full and unable to provide seating for people wanting to go 'up the line'.
With the new trains coming down for Paddington to Penzance workings, releasing some short HSTs, perhaps 2 car trains for local workings from Penzance to Plymouth, Exeter and even Bristol/ Cardiff will become a distant and unlamented memory.