Play every champion at-least once before playing ranked?

I've played thousands of games, and still have not played every champion atleast once (despite owning them all)

Not really. Just because I haven't played as Swain doesn't mean I don't know how to fight him.

I mean youd have a huge advantage to knowing what there kit does up front, but you wont know the ins and outs of it, like combos and stuff.

No Reading the kits of champs who you will be laning against is a start, then reading about every champ in general after awhile 125+ unique abilities and passives is a helluva lot to stomach in one reading and most people would want to take their time learning this game i'd imagine, not raging because they fed on a champ they were not only terrible with but just outright didn't care to play