mejai is conditional, i agree. at higher elo mejai's usefulness becomes even more conditional depending on what the other team is doing. but i do not agree with your reason for not using mejai.
you are saying that if you use mejai and get many stacks, other team will FF you during teamfights, killing you and causing you to lose stacks. but whether you die or live depends on your personal skills and positioning.
i have been first targeted in teamfights after steamrolling entire game. its not uncommon for the enemy garen or shen to spearhead past my teammates, followed by their own team heading straight for me. but all it takes is a good wall, glacial storm, flash frost, and a bit juking around everybody to survive. by then you are making the enemy team pick their poison: whether to keep killing you and risking their team getting aced, or ignore me and hope i dont kill them fast. trust me, no team will be dumb enough to chase you around while you cast CC on them, reduce your health to 0, then try to break your egg while your team is raping them. either way, they would have realized the next time around that targeting me first past other teammates is a bad choice.
(p.s. lol, everyone is posting their scores)