A late round pick (Second Day guy, I assume, but how late?) holding out until August might find himself without a job. That's way late in the season and every passing week gives a bubble guy the opportunity to prove his worth. They know what they have (with wood, at that level) with the bubble guy; the draftee is an unknown. How much PT can he show with a month or less of games?
What you're suggesting might work, but I would consider it a gamble. Regard instrux as practice with scrimmages; the season is games. Which would you rather judge a player by?
On the Fall plans vs. ST plans, I think rosters are penciled in for the Fall and confirmed or altered in ST. What kind of shape the guys show up in at ST is critical. We've allseen it on the MLB rosters, expecting a guy to have a slot on the 25 man who loses it in ST.