I assume you know the Highlander-format for TF2. If not I suggest you read the excellent article of Junta on the topic first. Still, in a nutshell: Highlander consists of a team of nine players, with each class represented exactly once.
Highlander would probably be an amusing obscurity if it wasn’t one of the few deviations actually used in the competitive scene besides the common 6vs6 format. This fact has promoted the Highlander mode to a legit format and has been bringing it increased attention.
The reason for that is easy to see: Besides the neat gimmick of having each class actually represented in the game this format also has the nice feature to present a perfectly even playing field (at least on symmetrical maps). The identical line-up guarantees even chances for both teams, with only skill and tactical aptness deciding the game.
However, I admit that personally I’m not too fond of the Highlander format.
One of the aspects I love most about TF has always been the fluid and dynamic usage of classes to adapt to the different situations offered within the game. I’ve labeled this the „key-lock-principle“. A given defensive posture or an offensive tactic (the lock) is encountered (and overcome) with the proper line-up of classes (the key), and this line-up may be changing constantly to adapt (and counter-adapt) to the different situations.
To me TF with its different classes is a toolbox. Presented a certain problem I will select the class most likely able to solve that problem. This decision will be influenced by seeing which classes are already used by the other members of my team. All this adds an additional tactical layer to the game a format like Highlander would not have; to constantly adapt to the ever changing challenges presented as well by the given situation as by the enemy line-up. This is a kind of challenge I find fascinating and a way of playing I enjoy.
Of course there’s a downside to this: The this additional layer of teamwork to shine a higher level of coordination is required. Otherwise too many players try to make the same adaption and potentially just shift the problem (e.g. the presence of a spy which suggests having a pyro and consequently 3 people decide to switch, therefore weakening another tactical aspect; or a map – like Goldrush, 3rd stage – which really benefits from an engy building TPs, but suddenly the team has 2 engies).
Having said all this: As with everything in life this should also be applied in moderation. I’m not a fan of major scout rushes, and teams consisting of 6 or 7 demos and soldiers are usually not fun as well. And as always when writing about the game my primary goal is to promote a game which is as much fun as possible for all people involved. As such I appreciate servers which apply reasonable class limits to ensure that a variety of class line-ups are possible without letting is get out of hand [1].
All the above should in no way devalue the Highlander format. It’s a legit and interesting format with its own strengths and weaknesses, just like any other format.
But personally I love my TF as this toolbox with 9 very distinctive and unique tools suitable for different tasks to choose and apply throughout the game.
It’s a kind of magic ... 
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