There are 3 situations in which one would likely buy 
in a serious game.
1: You are ahead, and you expect to be easily STUPID FAR ahead quickly.
2: You are behind so much so, that your only hope to GET BACK INTO THE GAME IS VIA LUCKING OUT ON SOME PICKS AND GETTING A BUNCH OF INSTANT AP
3: You are either 
or 
.
Point of post: Buying mejais while behind isn't idiotic. Just really desperate.
EndlessSorcerer2 votesIt's honestly easier to get it at this point.
You can just grab Dark Seal for 400G after your first item and sit on it.
If you get enough stacks or seem to be snowballing, you can upgrade it. Otherwise, you can sell it later for a minimal loss.
Divewing1 votesThis is more just because people think: Mejais when behind=bad idea.
EndlessSorcerer1 votesSorry, read that just after I had woken up; I must have missed the last line.