While that may be fine and dandy, you're taking an 18 y/o prospect and turning him into a 19 y/o prospect. Assuming you're working towards the 4 years of development before turning him into a BL player, he's now going to be 23 when he starts his BL career. If you go about controlling him for 10 seasons(3 min wage, 2 arb, 5 LT), he's started to decline(assuming position player) as he's 32 in his last season. If you do the 20 game hold thing, he's 33.
If the prospect is 19-23, it's even worse.
Not sure what you're gaining on the back end for position players.