You are better off having a player pick the champion they are best with, rather than picking a champion to match a team comp. Players that pick champions to match a comp probably aren't that good or won't climb because being mediocre with a dozen champions is not the same as being good with a few.
This applies especially applies to the Elo you are in (Silver, Bronze). Even in gold, you should still be playing the champion you are best with and not trying to make a specific comp.
A lot of games do not need to evolve into a "team fight" phase. Despite popular belief, grouping can actually lose the game if specific players/roles are behind. If my ADC is feeding their ass off, the last thing I want to do is group and put the nail in the coffin in the game by teamfighting as my ADC deals no damage.
And guess what? Most of the time low Elo games a player gets crazy fed from the bottom lane.
A lot of the times skirmishing, split-pushing, playing around objectives, and simply sieging, is more effective than grouping for a team fight. Comps don't even really matter until you get into probably high platinum and diamond. Champions largely don't matter at all in low Elo.
Besides, most of those players know how to play around specific champions. They understand concepts such as split-pushing.
I noticed that lower Elo players always want to group and teamfight, but most of them suck at team fighting. They hate it when people try other strategies to win games.
Recently, I played a normal game to warm up for ranked. I was 
and we were winning by just split-pushing. My ADC and my APC were both behind but they could wave clear.
The enemy had a 
and a 
.
So two hyper carries and enough saves to prolong any fight. My mid laner kept asking to group. Over and over again. I had taken all the tier one and tier 2 towers while they skirmished, wave cleared, and disengaged.
I finally agreed to group. Lost two team fights back to back. Got annihilated. Which gave them baron after the first teamfight and after the second one they pushed and ended.
The person that requested we group the whole game was Bronze 3 and in the post game lobby our opponents asked why our team grouped. I should have just trusted my own judgement, but he/she was complaining almost since the 20 minute mark. I just entered the game to warm up for ranked, so I didn't mind satisfying their demand to stop him/her from requesting it every 3 minutes.
Our opponents recognized that by split-pushing, we were winning the game through attrition. Two of my opponents were really baffled why we suddenly changed tactics and grouped. They were literally trying to get us to group the whole game so that we would fight on their terms.
Either way, that's just one rather extreme anecdote, but figuring out if your team should or should not group is how you get better as a player. The answer is not always as simple as "let's group and win."
