Are you sure you were playing in a Pathfinder Society game? I ask because PFS was intentionally designed to contradict every concern you stated above. It sounds like your game was being run by someone who wasn't entirely familiar with how PFS is supposed to work. Modules are specifically designed with sub-tiers so they are playable by a really wide range of character levels. For example, you can take a character on a first tier adventure if he's anywhere between 1st and 5th level. Furthermore, the modules contain rules for determining the average level of the party and using alternate encounter stats (number and level of monsters, difficulty of traps, etc) for subtiers within that range, so every encounter is balanced for the party. The only reason you'd need multiple characters is if you want to play in a really low level game and a really high level game at the same time, but wouldn't you want different characters for that situation anyway? Pathfinder Society was also designed with flaky players in mind. Every session is a one-shot adventure; you don't need to rely on people showing up week after week. However, you retain the sense of advancement between sessions, so it's win-win. Also, since it's all module-based, your DM can change every game with no loss of coherence. In fact, DMs get character experience for running games, so the DMs character levels up with the party and is thus ready to go for any adventure they play. If you're having these issues with your regular Pathfinder Society game, I'd strongly suggest sitting down with your DM (or local chapter) and reviewing the guidelines for how levels, tiers, and subtiers work.
