Not to mention it'd increase the difficulty a lot if it comes to reading enemy bases (if I upgrade all mexes to t2 but pause at 90%, either you look closely when scouting or you'd have literally no idea since they don't get gray'ed out) or the game in general.
At least this would be crazy to do, because mex would have still t1 HP until it reached t2.
Your mex would be in extrem danger for a little advantage, a little raid could cost you thousands of mass.
Our oppenent would think "this dude has only t1 mex, must be all stopped at ~90%",
let send some bomber.