(Just my opinion. In this area, one size definitely doesn't fit all.)
If a kid is succeeding academically and socially the reason for holding a kid back in eighth grade would be purely for athletic reasons. It is an undeniable truth that a kid will be bigger and stronger with that added year and gain the desired advantage in HS tryouts.
Conversely, the message conveyed is that athletics rule the roost; academics and social maturity issues are secondary. When the time comes to crack the academic whip in HS (when girls, driving, long showers , and other issues arise) which virtually all kids require, what will be the new message conveyed? Will the kid understand that the multi-month test prep is far more important than athletics?
Andecdotal evidence doesn't really prove any general principle, but the kids I knew from who were held back (strictly for the sports advantage) did play their sport(s) in HS and most were advantaged by the hold back (as strictly defined by sport performance). None played beyond HS; and none caught fire academically in HS. AND, their peers who moved on to HS in normal time, left their former peers to repeat that middle school grade. New friends I'm sure we're made by both groups.
College coaches do look less at projection and more to immediate potential impact; pro ball looks more at projections. Take that for what is is.
I wish I had the crystal ball which is needed to choose which path is better. Just understand the advantages and disadvantages in holding a kid back for purely athletic reasons. I'd add that if a kid is struggling academically, diagnosing the reason early and aggressively addressing it is far more preferable then believing a repeat of material from eighth grade will improve academic performance.