User blog comment:Alterrea/Adapting the meta/@comment-5761259-20130723100401/@comment-9245010-20130723115829

Yes, that is the main problem with the underused/unusual picks. Underused picks are meant to be on the specific post, but they are picked less cause of the current popular picks/builds/comps/gamestyle (just look at all the midlaners like Malzahar, Brand and Ziggs). Unusual picks are not in their intended place in the meta (like the Veig supp you mentioned, or the Brand supports, by the way Zyra was meant to be a midlaner too ^^). Cassio is not that great as supp, as she requires mana to spam spells, her E is perfetly bad for supporting, her Q has too high mana cost. Pre-6 her only nice spell is her W, it is a great are denial tool, with a really bad slow. Post-6 she gains her ult but it only stuns people facing towards her, so Sona's ult is better in this aspect. While there is some logic  behind her, she is outclassed by many supports, so she is not the best supp :)

What I see the main problem being here is that people "try to change the meta). You shouldn't pick something unusual just to do something new (at least if you are not trying to have fun in blind pick or something). In certain team comps, I can imagine Cass supp being nice, but in random matches, it may be benefucal to stick to the meta.