Here's why establishing a trip rig is really important:

There are still outlying trips made of leftover flying with poor credit. Due to the nature of PBS, this is a problem for all pilots (not just junior ones) since PBS's job is to allocate flying. It cannot build complete lines for junior pilots out of poor pairings since it will run out of work-days. Its "solution" is to start swapping pairings. A senior person might get a desired trip replaced with a crappy one, and the junior guy gets crappy trips and maybe a nice one... yet is still saddled with minimum days off.

Further, consider that the TAFB/Credit ratio you mention is a system wide average, encompassing all pairing lengths. Think about this: a good paying local will credit close to 1:1. Standups by their nature usually have at least a 2:1 ratio (the existing duty rig). A two-day trip somewhat less (but still pretty good) since there is only one night away in a hotel that counts toward TAFB. So despite the fact that the system-wide ratio averages close to 3:1, your longer trips end up far worse: A 20-hour 4-day might have a typical TAFB of ~76 hours and have a 3.8 ratio (and that's not optimal, but isn't a bad pairing). A really crappy one, of minimum credit days has close to a 5.0 ratio.

I think that establishing a trip rig is a big win, and something that can be negotiated up in subsequent pay agreements.