great example of why it's hard: my 3B just got injured for 12 days. i could start his backup (who's out of options) or i could call up a guy who's been tearing up AAA (.355/.425/.737, 7 homers in 78 ABs), who has one option . i want to eventually start the AAA for a few seasons, because i just traded for a High-A SS and plan to move my current SS to 3B when the young guy is ready.
if i'm going to bring up the AAA 3B when the rosters expand to 40, i might as well do it now. if i want him cheap in season 4, i shouldn't. if i promote him, i also have to promote a guy to play 3B for the AAA team, and i don't want to rush my younger IFs.
and that's a sign that this game is amazing: my 3B gets injured, and it's fascinating.