Except that w/ only two roles that is pretty much statistically invalidated. I've almost never seen 2 adc bans in the same game, but let's go w/ 4 to be safe, you'd need 6 ADC champs to be guaranteed the ability to have 1 up (let's call it 7 bc maybe there is a miracle and 4 are banned, one is taken top or mid, and you pick after enemy adc), if your other role is support then you can get away w/ 6 or 7 also bc similar logic. So at max you need 12-14 assuming just god awful conditions. Realistically you'd need way less, maybe 4 or 5 ADC champs and 5 or 6 supports.
The thing is when you're drawing from the entire champ pool there is a chance that is much higher hat all 6 bans may freakishly hit your pool. However, there is almost a 0 chance that they all hit the same role (even when it was fiora, garen, morde, darius, skarner every game, skarner and morde were different roles, so even if you played top and then went jungle or adc for your other role at max you hit 5 bans against you). There is more of a need for multiple champs if you play roles that have over lapping pools (for instance if you play mid and you play annie, she can also be picked at support so there is a chance both mid and support pick champs you own even though you only on mid/top champs or something). However, the limit to two roles removes the need for 16 champs if you only have champs that play those roles because there are guaranteed to be other picks that aren't in your champ pool (ie if you play top/mid then even if one team runs a kill lane bot there will be one ranged carry in pretty much 100% of ranked games played, so at max you'd need 15 instead of 16 champs, and realistically less than that w/ almost any combination of viable top/mid champs). The current system has a very very very low possibility that 15 champs could match up to your exact pool, but if you have champs for two roles and are guaranteed one of those 2 roles then you could easily find champ pools that would never be ruled out over every single game played in ranked on every server for a month because it's only 2 roles and other roles won't over lap in the pool.
