In fairness the 15★ weapons from collection folders probably aren't ones worth taking to +35 anyway unless you really want a Mastery V augment factor for some reason. Mostly they're there to be a stepping stone (at a time when we've had multiple better 15★ weapons given out by campaigns) or for alt characters to use.
Arguably it's 14★ egg sheets that are more worth doing multiple times, as it's the only real way to get Casting Spirit Cookie and most (all?) pets need three of those. Mitigating factor: You can just do the luminmech part of a sheet, collect the cookies and then cancel and immediately cancel and restart the sheets to skip both the UQ portion of the sheets and their 2-day cooldown.