I think it would be handy to use them as external controllers, but with a couple caveats:
1. I hope they add back in the analogue triggers Nintendo introduced in the GameCube and then ditched. It's so much better using an Xbox controller for driving games because of its dual analogue triggers for acceleration and braking.
2. I hope they redesign joycons to be more comfortable for a wider range of hands, especially larger hands. The switch Pro controller is great, but the joycons are the most uncomfortable controllers Nintendo released since the original NES/Famicom controllers, and significantly worse than even them when using a single joycon. I understand they were trying to find a compromise that allowed them to fit onto the side of the device without adding too much bulk, but they failed in a way that shocked me after generations of carefully crafted ergonomics from Nintendo.
3. I hope they redesign the form factor of the switch to be more comfortable in handheld mode, even the WIi U pad was more comfortable, and it's significantly bulkier. If they do this, I'm happy for the old joycons to not clip onto the sides of the new system but just work as externals for games not requiring analogue triggers.