Each game the pickup player plays increases his progress toward a season. Same is true for a regular player.
I suppose the pickup player might have a knowledge advantage over backups who don't see the field a lot on regular teams.
However, the pickup player will have mostly seen random plays instead of stacking the most common plays.
Seems mostly a wash.
Most starters on a regular team will have a knowledge advantage over converted pickup players, both in terms of quantity and quality.