[NoOne]

mlb is a better development environment for any player *nearing* fully developed. they don't even need playing time to benefit from it (unlike minors).

where that line is drawn is the opinion part without data to back it up.

just a loose guess: if AAA is ~80% developed or outlined by sabermetric pcms?? then anythign >80% developed likely draws back down if left too long in AAA - excluding rehabs?. some "glass ceiling" due to AAA vs MLB that affects development -- or the % chance of positive development if not a hard ceiling based on proportion of the player's development. however it occurs, you probably don't want fully developed player stagnating in AAA for long.

whether you use him depends more on service time, like tgh said.

if they are going to be good players, i like to get their careers started ASAP, but if their current ability is barely mlb-baseline, i'd prefer to go with someone more expendable as an injury replacement.

if the guy can be inserted 1-5 and be a key contributor, that's a different choice for me even if they get some service time.

the way the math work on it, as long as you weren't intending to short-day the guy his rookie year, it won't matter in most cases if they get a month or 3 in beforehand.

you accumulate a year of service time only after the regular season and only if it adds up to the minimum days -- and never more than 1 year at at time.

it does carries over, but it doesn't always add up to another year... e.g. let's say "180" minimum time. a player has 300days at the end of year 2, but will only have 1 year of service time. (1 year and the extra 120days that don't mean anythign when it comes to financial stuff)

(same exmample #s) if they only get 60 days in year 3 due to injuries, that's an example of the rare instances it can hurt a bit, financially. they'll gain that 1 year back just be completing ~60 days (120+60=180, so 2 years, now) without those extra days they'd still be 1 year of service time.