[euccastro]

You thus can't follow the recommended typing practice of pressing a modifier key with the opposite hand as the key being modified.

One of the reasons for that practice is that modifier keys are typically pressed with your pinkie. You also use that finger for typing characters, so for these characters you need to break your habits. Eventually you build new ones, but it'd be better not to need this. Even for characters that you don't type with your pinkie, you may need to stretch your fingers in awkward ways.

Thumbs are only used for the space bar, which is seldom used with modifier keys --and you can use either thumb for it, so you can alternate. Also, thumbs are less 'coupled' with the other fingers.

I'd still prefer my modifiers duplicated on either side, but I can see how this may matter less if they go under your thumbs.