Its not much of an advantage if you two are starting at the same game knowlwadge. Runes advantage is kind of non existing when you both know little of the game. Either way, you will get Runes later (by the time you play ranked) so its not big deal in the long run. And definitely not what Ben have insinuated.
Runes is going away anyway, so that point is moot.
Runes are going away and thank god for that.
And yeah, it does give an advantage to have more access to champions.
Jimmy Spoils has been able to try out 30 different champions because he has purchased many of them. Spending atleast 3 games with each, he knows these champions and what these do. Therefore, he has less of a burden of knowledge than Bobby Poor, who only has 15 champions in comparison, has. Bobby Poor may have 6 games per champion, but when he goes up against a champion he has never played nor seen, he does not have the knowledge Jimmy Spoils does when it comes to knowing how to play against these characters.
League is a game who's barrier is burden of knowledge. There is only so much you can learn playing against champions, and only so much you can learn from playing them. To have access to both possibilities at any time, by having a wider pool of champions, means that the person who has more champions has a clear advantage over someone who doesn't.
After a point though, of time spent by playing league, this advantage curve flattens out, and this is where personal skill starts coming in. Some people plateau, some don't. There are downsides to rotating who you play as constantly, compared to someone who OTPs a single champion.
But, this is a big but, if someone joined league today, it would be a very long time to get the knowledge down, and that's one of Leagues/Dotas biggest problems. As the game expands and changes, it becomes harder and harder to get at a feasible level.