There are three different types of players: The player who mains a role, the one-trick pony and the jack of all trades. I will go over each one and discuss the positives and negatives.
Let’s start with the one-trick ponies. The positives of playing only one champion are: being able to always dominate the game when ahead because you know everything about your champion, like exactly how much damage you can take and dish out at every point in the game; winning or staying even against matchups you shouldn’t be able to. The negatives are: being near to useless when you aren’t on your champion; a reduced overall skill level due to being unfamiliar with how other champions and even lanes work. There are people like Annie Bot and InvertedComposer who are very successful in solo queue and if you really enjoy playing only one champion then go ahead, however, if your goal is to develop and improve as a player, then being a one trick pony isn’t the way to go.
On to the people who main a role. These are the most common type of players. The positives and negatives here are almost the same as with the one trick ponies, because if you get your preferred role than you’ll perform at a high level and if you don’t you’ll be a detriment to your team. The rule of thumb to counteract this is by being able to play two or three champions at the other four roles so that when you don’t get your main role you’ll still be able to do something. This is a solid model, assuming you can play two to three champions in each of your non main roles, but it isn’t the most optimal for improving.
Last but definitely not least, are the jack of all trades, aka the fillers. These people are very rare and we all know that delightful feeling we get when you jump into champion select and someone types filling. Being a filler is the best way to improve because you gain exposure to a lot of different champions and their different play styles as well as the perspective of playing all five different roles. Your game knowledge will increase tremendously and your mechanics will improve as well.
Being a filler is by no means easy, however, you don’t need to become a “full-time” filler to improve your game. Start playing normal games initially and every time you get in a game let the other players chose their roles and you take what’s left. After you’ve done that a couple of times you can move on to team builder if you have a specific champion and role you want to try. I’ve been doing this throughout the pre-season and I can honestly say that my game has improved a lot, both in terms of game knowledge and mechanics. If you don’t like it, then don’t do it in ranked, but even if you do it only in normal games, you’ll become a better player.