If we got rid of tiers everyone would either be using a team of Ubers (assuming getting rid of tiers also gets rid of the banlist) or a team of OU Pokmeon. The reason they are OU is because they typically do their jobs better than a UU Pokemon that does the same job. So getting rid of tiers essentially forces everyone to either play Ubers or OU (again depending upon what happens to the banlist).
So instead of being able to use the slightly less useful (but usually more fun) Pokemon in a UU tier battle, everyone will be fighting on the same terms which means UU Pokemon will be competing against OU Pokemon in every battle. While a good UU team can beat an OU team (or benefit from the addition of some OU Pokemon), that doesn't add anything for the community. If fact, it limits the battling options. You'd basically create a bland metagame of nothing but OU battles.
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