I've built a number of bikes on 1980s steel frames, including a couple of Traveler frames. Right now my single speed is a Traveler frame, 22 pounds total, including rear coaster hub. It's my comfort cruiser, for fair weather commuting, getting around town, recreational rides, going to the opera, etc. The weight of the old Schwinns was not just the frames, but was spread throughout all of the parts. There are a number of things that I look at when I assess an older bike. Probably two major issues:
1. Are the wheels in good shape and do they have alloy rims? I see no point in riding or maintaining steel rims. You know that 27" tires keep getting harder to get.
2. Are the brakes good enough? Those old Dia Compe can vary according to their condition and upkeep. Some have been good as new, others mushy.
Perhaps the biggest headache is accommodating modern, wider tires, and fenders. Many of those old frames didn't have enough clearance so you're limited in your tire choices.