You'll need to use motion controls to catch Pokémon in Pokémon: Let's Go Pikachu and Let's Go Eevee, regardless of whether or not your Switch is docked, and they're compulsory throughout.
There was a suggestion this was the case from the off but, in a statement passed to Eurogamer, we received the following confirmation - and clarification - of how exactly the new catching system works:
"In handheld mode with both Joy-Con attached - you are still required to move around to aim, but you press a button to throw the Pokéball.
"It is not touch screen like on a smartphone because the Switch is quite a bit heavier than your smart phone."
In other words, if you're playing Let's Go out and about - or just handheld at home - you'll still need to wave your Switch about to aim and catch Pokémon. It's just the press of a button that actually launches the ball, presumably once you've lined it up.
There is some precident for this kind of thing - certain Shrines in The Legend of Zelda: Breath of the Wild come to mind - but it's certainly the most notable, recent use of the Switch's gyroscopic feature we can think of here, especially given there's no way to work around it.
In other news, The Pokémon Company has confirmed the region of Johto will not be a bonus feature in the game (as it was back in Gold and Silver).
Earlier this week, The Pokémon Company confirmed you will need a Nintendo Switch Online subscription to use Let's Go Pikachu and Eevee's online battling and trading functionality.