Heh. You can’t know whether it’s worth importing until you’ve already imported 
Well, there’s the distinction between “imported” and “added to timeline”. Importing it just adds it to the database, but in a “disabled” state. Then it’s a matter of whether you actually want to use it or not, ie add it to the timeline or not.
I’m mostly only adding to timeline the workouts that have a cleaner / more complete looking path line on the map versus the Arc recorded data.
In practice that means I add the short 2-3 minute walks from home to Starbucks each day, because Arc’s data is always a bit poor right after waking from sleep, so for a short trip like that the first half of the trip is sparse in samples and not terribly accurate. Where as the workout (I start a walking workout on my watch before leaving home) will have full data right from the moment I start the workout.
There’s a fairly typical example. The green being the imported workout, with (mostly) proper data from the start on the right, while the grey is Arc’s data, which is… okay in this case the Arc data follows the road more accurately. Perhaps not the best example.
But yeah, I find it especially useful in those cases where Arc waking from sleep meant the start of the trip is data-sparse, where as a manually started workout doesn’t suffer from that slow/gradual wakeup.
Outside of that, I’ve been finding that the Arc recorded data is usually better. Even though there’s more samples from the workout data, it’s more aggressively smoothed than Arc’s own data, so it’s often a less accurate path line, with sharp corners too aggressively smoothed out, etc.
I’m curious to see whether that’ll change with the Apple Watch Ultra and its dual band GPS. And other wearables that record workouts might be producing less aggressively smoothed paths, so might actually be beating the Apple Watch already on this. Though I’ve no evidence of that yet - just a hunch.