User blog comment:TheTobarMethod/ADC Mid....It can work!/@comment-5514182-20130128231744

Alright. ADC mid is actually an old tactic, and it had the same reasoning you did, to gain levels  faster. The problem is that ADCs scale with gold, and not levels. You'll get as much gold mid as bot, but a mage would benefit more from the extra levels.

If you still had ADC bot w/ support, you'd sacrifice the burst damage from mid. You could have an AP jungle like Diana and Fiddle, but they'd be deprived of both gold and levels. You could have an assasin top, like Akali, but you'd lose some tankiness. You'd also have no magic damage if you have a tank jungler and bruiser top.

On the other hand, the 'new' Meta of stacking health and melee assassins mid allows ADC mid to be quite viable, but only in those situations. Generally, mages will beat auto-attacks in lane, just saying.