Many of the "mid-tier" talismans are still quite rare, but are very valuable in many game modes (Shell, Immortal, Strike, etc). We definitely don't want people to crank out a dozen sets of these in a single weekend, so they're tuned to take quite some time to get a full set of. That said, we already increased the frequency of some of these exclusive talismans so that players can get more of their collection to be competitive quicker, and we're pretty happy with their drop rates right now.
The absolute rarest talismans (Quake, Lightning, Rage, etc) are obviously not nearly as viable for PVP due to the prohibitive power cost and the need to dedicate so many slots to a niche effect, but will help in PVE game modes (and possibly future game modes) where building up to their crazy high power cost is more viable. These are tuned to be even more rare so you have something to work towards in the really long term, so I wouldn't want you to consider them as "something to grind for", but instead something to slowly earn a full set of over time as you cleanse the Brave New World for the sake of other rewards (or to capitalize on events).