I think I know what was meant; why not have a mixed chassis army with the proviso that you only fight a single chassis battle. This means that you can chose the right chassis for the right battle.
The answer is of course that when it comes to large, or collaborative battles, then you suffer. This is where the single chassis really comes into its own.
Also, if you are fighting on several fronts, you may have different sections of your army far apart, meaning that you use a less than optimal section of your army, or waste a lot of transport resources.
I suppose it depends on what position you want to finish in. If you intend to have dozens of ships, and the ability to crush strong players, then you must go single. If you just want to squash a few new players, then it probably won't matter so much.
I can see a limited application for mixed chassis approaches, but only with a couple of mixed squads. Say 95% single and a few varied just for minor conquers (especially early in the game) For example, with an inf chassis with say 20 inf squads, why not have 1 squad of slashing heavies. This is used only for minor skirmishes where the players almost always have anti inf inf. Everyone already does this to some extent with spam squads. This is just one step up. 1 squad of this type will take out 2 of what it is likely to be up against.
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