1. I honestly have never heard of people buying Sheen or TF for the mana (I think it's bought for the Tons of Damage), except for Ryze buying Iceborn Gauntlets. Manamune is barely bought at all, in most cases. Often, again, by Ryze.
2. The attribute salad, at 7, is almost identical to Athene's' at 6. Take out a single attribute, like the 5% ArP, and they'd be even (items usually improve upon every stat found in their component parts, and 5% ArP seemed more effective/engaging than just adding an extra 5 flat ArP). TF has 9. Gunblade has 6, Hydra has 6, a bunch of items have 5, and so on.
3. A mechanic like Mana Siphon is basically lifesteal, only it scales on mana instead of attack, and restores mana instead of health, and (importantly) only happens when attacking champions. You can't sit in brush or at tower and watch your blue bar fill up, not even if you're poking minions. Also note that it's supposed to replace Mana Font AND the 12% restore on kill/assist, which, in a large, successful teamfight, can restore 5-700 mana on its own in about 10-20 seconds (equivalent of 25-70mp5). Finally, Athene's makes additional mana and mana regeneration obsolete as well (even when not in combat with an enemy champion, to some extent).
I just wanted a mana booster for physical champs that more people might actually buy. =\
It would certainly feel more fun to me than repeatedly ending matches with a barely-half-full Tear/Manamune/Staff.