Yep, rusted parts are worth a bunch because they are needed for some special DIY(s?) like the Big Robot you can purchase at the Nook Terminal. You can only get them after each time Gulliver appears on your island and because you need 30 for the Big Robot DIY, it could take months and months to save up for non-TTers.
They're not widely useful, but they're important for at least one rare item.