At the airlines I've worked for, a secondary line made of "scraps" is still a line. If someone is inconsistently getting a published line and other months getting a secondary with flyiing, then I would call them a line holder. They're not on reserve. How or why that happens isn't relevant if it happens all the time.
Peak is a factor in about 15% of our year. Are you going to discount the other 85% because things change during that brief period? I don't understand how a line holder in non-peak ops can't continue to be a line holder during peak (unless they have to be able to hold Xmas off in Dec to still be considered a line holder). In my experience during peak, non-lineholders become lineholders if they're willing to work when everyone want to be off. But, if your definition of a lineholder means they can't get scraps or "crappy" lines then I guess that wouldn't count either.
I just flew with an F/O whose been here for 18 months. Hasn't been on reserve since last fall. He's getting "crappy lines" or scraps via secondary, but he ain't on reserve. He can trip trade, bank all the BKO hours on the long haul and benefit from every flavor of disruption we have. So, I'd call that a line holder. On the 777 a crappy line usually means F/O flying trips with no front or back deadheads and a few multi-leg days of hub turns in Asia or Europe. Still not too bad - they're just not getting the DHs or nice RFO trips that average 8-9 CH of pay per day. I'll bet there are plenty of FOs who would much rather do that every month than commute to reserve and maintain a crash pad whether they're considered line holders by everyone here or not.