If for no other reason, play the highlander format because it is entertaining. Both players are restricted to choosing only one of a given thing, and so have new challenges to face on the tabletop. The Dark Eldar player with six venoms is going to have to stretch their tactical abilities by trying to make do without them. It is possible that both players can see this as a stress-free 'fun' game where the winner doesn't mater because you have placed artificial restrictions on yourselves, lightening the mood and playing more to 'see what happens' rather than win by superior listbuilding. Conversely, you can still play it competitively, but the mixing of units will still allow for more tactical diversity and generally results in a more enjoyable experience from what I have seen. More ways to play and more variety of playing experiences can prevent stagnation and boredom that comes from playing yet another rehash of the same old game ad nauseum.
Rare Units and Models
Ultimately, highlander format forces some rare units on the table. Generally seen as sub-par or even 'garbage', certain models almost never are actually fielded in anyone's army. Maybe it's a unit you bought because you liked the look or fluff, or perhaps it's a model that was useful in a previous edition, codex, or ruleset that now finds itself diminished in power or too expensive pointswise to be of any real utility in your list. With highlander, you're more likely to use it simply because you have less of a choice to fill its role with something 'better'. This serves several purposes - you will make use of the toys you bought and painted, be able to try different things on the tabletop than your usual list allows you to, you will see different enemy units than you usually do, and you will not be as severely hindered by choosing suboptimal units because your opponent will be doing so as well, leveling the playing field to some extent. These all can make for a more fun and varied experience without diminishing too much about what we love in the game competitively.
I suggest giving higlander format a try. If nothing else, it will be a novel experience, and it may just become a standard gaming format you add to your rotation when meeting up with friends to roll some dice.
Have you played Highlander before, or seen it in action on another person's table? Let me know how it went int the comments!