Let me sell you not banning or playing Syndra/Cassiopeia.

Syndra:

• Everything except the ultimate is a skill-shot, meaning she can be easily dodged. You don't want to play something like that, and you don't need to ban something like that.

• She has no mobility. Mobility is good. You don't want to play an immobile champion, and you don't need to ban an immobile champion.

• She's just a bad champion, there's no need to play or ban her.

Cassiopeia

• Without landing her skillshot, small target Q, she's useless, meaning she can be easily dodged. You don't want to play something like that, and you don't need to ban something like that.

• She has no mobility, outside of landing her difficult Q, and even that's bad mobility. She can't even wear boots. Mobility is good. You don't want to play a champion like that, and you don't need to ban a champion like that.

• She's just a bad champion, there's no need to play or ban her.

In conclusion, do not ban these champions, and don't feel obligated to play them either, unless you really want to. They truly are a waste of a ban, so yeah, don't do it.