Ever since most people can remember, there have been two dominant formats in which most competitive players participate: 6v6 and Highlander. I believe that this arrangement has been ultimately detrimental to both formats and to the general progression of competitive TF2.
The main problem has been and always will be that having two formats effectively splits the scene and its resources between the two. Regardless of how one may be considered the premier format and which one top-level play actually occurs in, the fact remains that the other will still receive significant resources that cannot be used to improve the other. The most significant of these is players - many players cannot commit to the time commitment of playing both game modes, so they must pick one or the other, naturally leading to lower numbers for both formats. These lower numbers make both formats less desirable to potential investors in the game, as a smaller scene represents much greater risks and lower returns. It also means that certain talent may be restricted to one game mode or the other, resulting in the quality of games not being at its potential maximum.
Highlander is generally recognized as the lesser of the two major TF2 formats for several reasons. First and foremost, the number of players required for each team is too much—nine players simply aren’t feasible to consistently coordinate together for events. In addition, having each class represented once and present all of the time also promotes the idea that all classes must or even can be useful all of the time, something which simply isn’t the case. An analogous situation in Counter-Strike: Global Offensive would be requiring teams to have each player use a different weapon class—clearly ridiculous. This also restricts the development of the metagame, since the only differences that teams need to expect are in weapon choices, most of which tend to be situational. Of course, Highlander has gained much popularity since it is promoted as a learning format that is much simpler to enter for inexperienced players, and has brought in new players to the general competitive scene by doing so, but it is unlikely that this is an overall benefit in comparison to the division caused in the scene.
6v6 has continually maintained its position as the premier format of TF2. It does not suffer from the same fundamental problems that Highlander does, since it only requires six players per team (one off from the standard team size of five for competitive games) and is not fundamentally defined by a strictly defined class limit as Highlander does, so it is significantly more suited to be the major (and eventually exclusive) format. However, Highlander’s presence has led to different problems. For one, lesser consideration of newer players has allowed the format to evolve around a certain view of the game, and its ruleset (most significantly class limits and weapon bans) enforce such a view by restricting anything significantly impacting that style of play. This leads to an outsider viewing such restrictions as purposefully designed to promote a standard lineup (two scouts, two soldiers, one demoman, one medic) even if such a lineup would be ideal without any restrictions whatsoever. Also, as discussed previously, the format receives a lessened flow of new players since many are directed to play Highlander as an easier format. This slows the overall growth of the format and prevents it from evolving as quickly and well as it could.
For the past few years, the situation has been at a stalemate, with 6v6 being the more recognized format but not able to claim dominion over all of the competitive scene. This is now on the verge of changing, as Valve is moving toward in-game competitive matchmaking primarily focused on 6v6. Official support in this manner will give the format much greater legitimacy and draw in many new players. It is my hope that in deciding the finer gameplay settings of matchmaking, Valve will choose to move forward with no class limits and no weapon bans. If they do so (and encourage leagues and other competitive events to follow), it will remove a major perceived barrier for new players entering the scene. In addition, it would allow all types of strategies to be tested, with undesirable ones being flagged and properly looked at. In this way, the method for dealing with broken strategies would shift from banning to rebalancing—something Valve has to be quite adept at in the past year with their patches during the Gun Mettle and Tough Break updates, and certainly a more positive way to deal with such issues.