OP is right from a long term perspective. I would say you probably want to farm at least 1 refresh in the mod challenges each day if not 2 (or more depending on what characters you want from cantina).
The 100% drop rate really makes it much better. You will get those speed arrows that are pretty much required on 95% of your toons. 8, 9, 10 speed secondaries are not that rare either. While these aren't too useful in arena late game they are better than nothing AND useful on your b squad toons for raids/gw/events etc.
That said... CLS could be a big game changer, if it takes 2 weeks to farm toons/gear to get him isn't that big of an opportunity cost. Worst case scenario you have a good RAID leader most likely.