No they don't. The experienced 1/8 brushless folks run single packs - the cells are all matched within that pack (only 1 matched pack vs two matched that have been shoved together).
Downsides of higher voltage. You might need a better ESC (a factor if you get a Novak or LRP), but as you want either an MMM or RX8 this won't be a problem.
The packs are a different shape - the higher voltage lower capacity packs tend to be shorter and wider than the lower voltage higher capacity packs. Whether this is a disadvantage is up to you.
Charge times.
Going back to my 5S 3300 and 3S 5500 example - it still relies on power output - IE - you end up with the same requirements power wise. However, you don't need a charger with as much current to charge the higher voltage packs faster (assuming the power is adequate). To charge a 5S 3300mAh pack at 5C, you need a charger capable of 18.5V, 16.5A and 346.5W (21V x 16.5A) - readily available. To charge a 3S 5500mAh pack at 5C, you need a charger capable of 11.1V, 27.5A and 346.5W - very hard to come by.
With chargers, the current is often the limiting factor. So, in reality, the same charger will charge the 5S faster than the 3S.
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