Why purchase Mejai's Soulstealer on supports?

On mid laners, this item is acceptable because they can afford to take risks, and since they can farm, it isn't as much of a problem if they are purchasing cost-inneficient items (Mejai is cost-efficient with the initial 5 stacks, but if you can't stack it, then it loses efficiency). They are, after all, one of the main damage sources from the team, and as such, they need to get as much damage as possible and Mejai is extremely good on that regard if you manage to fully stack it.

What I can't understand, however, is why there are people who purchase this on supports. Mejai is one of the worst items to buy on a support, even if he/she has abilities scaling with AP, for a couple of reasons.

The first; it's risky. As a support, you can't afford to waste gold at all; support is the role who has the lowest gold flow. If you can't manage to get stacks and/or you lose them, you just threw away a lot of gold that you could have used to purchase other, better items.

The second; to be able to mantain stacks, Mejai requires that you don't die. This contradicts with one of the support's main goals, which is to die for the team, if necessary, to protect the damage sources (ADC, mid laner, etc) so that they can win the fights. Of course you should try to stay alive if possible, but most of time a dead support is preferable over a dead ADC/mid laner/etc.

The third; Stacking Mejai on a support takes longer, assuming that you don't get any kills, due to the fact that assists only grant 1 stack.

The fourth: there are other, much better AP items, to buy if you plan to build AP on a support. For example, . Not only it is much more cost-efficient, it also helps the team, due to the magic resistance reduction. You are also guaranteed to mantain the 70 AP no matter how much you die, and the magic resistance is useful if the enemy AP is fed.

The fifth; Even when fully stacked, Mejai's Soulstealer isn't always as benefical as it seems. Most "AP supports" actually have low AP ratios on both their offensive and their defensive spells, so it's likely that the you aren't fully taking advantage of the enormous amounts of AP granted by a fully-stacked Mejai. And if you plan to use it for the cooldown reduction, there are other better items for that purpose.

I go mad when I see someone that says they are going support, then they purchase Mejai and don't even purchase a . Yes, everyone should be warding, but this by no means is an excuse to not ward if you are going sup.

I say that because on my original server (BR) most people do not understand this. I just created a thread explaining how is a bad purchase for supports outside of a few, specific situations. People didn't understood that and I received insults and I was humiliated.