Remember, competitiveness isn't binary; it's a spectrum. On one end you have silly servers where the objective is just to fool around and pose for the camera. On the other end, you have big invite tourneys where real money is on the line. Most players, servers, and communities fall somewhere in between these two extremes. The problem with lobbies, be they the standard 6v6 or the less popular Highlander is that they're played with artificial restrictions in place. TF2 was designed as a 12v12 game; in a small game like 6s, the core classes (Scout, Soldier, Medic, and Demoman) are overpowered in such an environment to the point that the meta has solidified around them. Even in Highlander, forcing a team to have a Spy means that you always have to worry about Spies, whereas in a normal pub game, a team might not have a Spy so the spychecking done by the enemy team is a waste of time (not that they know that).
I come from an RTS background, and the thing about Spies is that like stealth units in RTS games, the threat of having them can be a greater weapon than actually deploying them into battle. For example, if you are known to always use Dark Templars or Banshees, your opponent can blind counter you by building detection. Also, if you always know that DTs or Banshees will be present, you will play accordingly and the surprise factor that these two stealth units have is lost. However, if you don't always use them, your opponent has to scout or guess if you have them. In TF2 terms, spy checking is a waste of time if the enemy team never has Spies; the potential threat of instant stabby death is greater than the actual stabby death itself.
Other support classes also suffer in competitive formats. Engineers are an awesome support class if you already have a good core lineup to support, but they're dead weight when you have a smaller player count and every player is needed to contribute now instead of later (the Engineer is a class that gets stronger the longer he stays within a specific area). Pyros are pretty much dead weight in a format where you can't afford not having someone that can actually fight directly with the enemy team. There are also other things that work well in 12v12 that simply doesn't work in competitive formats:
1. As an Engineer player, I can place a sentry gun somewhere it will shoot down incoming Soldiers, Demomen, and Scouts, as well as flanking Scouts and Pyros. I've gotten long killstreaks with pub-optimized strats that pro and semi-pro players never have to deal with. It's not that these strats are cheap or cheesy, it's that they simply wouldn't work in a competitive setting where having 1 perma-Engineer means an insta-lose against competent teams.
2. Full-time Pybro is viable in a 12v12 setting. I need never venture from our battle lines; I simply spread happiness to my team and occasionally catch people who aren't already happy (enemy Spies). I airblast enemy projectiles and burning teammates, and unsap friendly buildings. By doing this, I can keep my teammates facing forward and firing forward instead of wasting their time and ammo spychecking. This is one thing I like to do on TPF servers, where competent Spies and competent teammates abound.
So yeah, please consider hosting a competitive pub server in addition to your organized lobbies.
