Then there's no way to determine which team options are meant to apply to each player, such as name colors, friendly fire, visibility settings, etc. If you intend to have a "preferred team" that a player chooses, then there's no point in being in the other teams. Scoreboard scores are already well available and can be used instead for that type of differentiation.
Minecraft-things: http://skylinerw.com
More Minecraft-things: https://sourceblock.net
Guides for command-related features (eventually moving to Source Block): https://github.com/skylinerw/guides
I primarily hang out in the /r/MinecraftCommands discord, where there's a lot of people that help with commands: https://discord.gg/QAFXFtZ
Their corresponding subreddit: https://www.reddit.com/r/MinecraftCommands/