I can name several anime I think could have been longer which are the following: Project K, Zetman, Nyan Koi, The Sacred Blacksmith, Darker than black meteor, Omamori Himari, Angel beats, Night Raid, Black Butler 2, Psychic
Detective Yakumo, Another, C, Blood-C, No.6, Dantalian no Shoka and many more.
Granted though the only benefit for doing short anime series of 12-14 episodes or less is being able to chuck out new anime faster. However, like TheAwesome said, doing short anime make the plot feel rushed. Ideally I would love to have about 26 episode to
120 episodes depending on the anime. Anything higher than 120 episode is pushing it for me at least. I don't think anime is going to stop using 12 episodes or less unless more people complain about it or stop buying products.