"Is it worthwhile to have a ______, or just disc up/down to use the ____ or ____?"
Regardless of what type of disc is under consideration, I think it's important to approach this problem in the same manner. A few questions to ask yourself when deciding whether to add another mold to the bag:
1. How comfortable are you powering discs up/down, vs trying to replicate the same power on all throws?
2. How comfortable are you manipulating release angle, vs letting the disc's natural stability do the work?
3. How comfortable are you adjusting throwing height to control distance?
4. Do you play in the wind a lot?
If you can accurately adjust 2 out of 3 release variables (power, angle, and height), then generally speaking you are better off not adding an extra mold to the bag. Now that is assuming you already carry a relatively full assortment of discs, and are just debating "how big of a hole in the bag is too big." The one big wrench in this mental process is wind. Playing in high winds dictates what angle and height trajectory you should release the disc on. If you regularly play in windy conditions, then I totally understand wanting to have a bunch of options on the extreme ends of the stability spectrum (both really OS and really US discs).