Personally I play unranked simply because I'm super casual at SC2 these days (kid/job etc) - I don't really want to practice/improve, and ranked play in any game tends to force me into that mindset. The lure of competitiveness is strong.

I really was low masters/dia (dropping to dia mostly due extended to off-racing - different MMR/ladder entry per race plz bliz) in WoL and half of HotS before I flat quit SC2 and probably played 50-80 games a week - and now in lotV I mostly play against plat players, the odd dia (which i maybe win 1 in 3 of) and maybe play 10-15 games a week.

So it's not like I don't know where my MMR/ladder ranking would fall (somewhere in mid plat).
I suspect everyone who plays unranked knows what league they'd fall into.

I simply don't play ranked though as SC2 is addictive, and resisting the urge to sink more hours, train harder and push higher would be too much. I simply don't have the time for that, so like any temptation it's easiest handled by removing it.