Is it a good idea to invest time in the Unique card reward for a complete collection?

In the game Evilibrium you control creatures which battle and gain levels. You can also evolve creatures to a next level. There is also a collection ingame, and if you get three stars on a whole line of creatures, you'll get an unique card. The requirements for three stars are the following source:

• the creature has been in action – you must fight a battle, using the creature in your troop;
• the creature has been upgraded to maximum; after being evolved, the new creature -receives a half of gathered experience points;
• creature's skill has been upgraded to maximum.

It can be a bit confusing if you don't play the game, but it's very well explained in the beginning, so I understand that part (and any beginning player too, if he/she is following the tutorial).

Now is my question: is this wise to do? An unique card is very awesome and good, and cannot be obtained in any other way. But you have to spend a lot of time with lower ranked creatures. I am currently playing with a Common card (the lowest rank). The max level is normally 40, and I have been playing quite some time with it and it is currently level 23. It takes a valuable space in my load-out (1 of 5 places) and I would happily trade it for a Rare card. Is it wise to continue playing with this common card (and after that the uncommon evolvement) to obtain the Unique card, or am I better of equipping the highest possible cards, and farm crystals and buy creatures?