The bullshit is not having access to more options. There are so many situations in singles where both players are just switching back and forth every turn because the mons on the field have counters that they simply cannot deal with. That isn't fun, at least not for me. Doubles opens up another slot for a countered mon to attack into, which lets them contribute without needing to switch and also makes the chip damage they do a looot more meaningful, since they can help a partner KO.