Many know after reaching level 30 wild Pokemon have capped CP potential. After level 30, boosting the CP of Pokemon is dramatically decreased. For example, a Snorlax maximized at player-level 30 and a Snorlax maximized at player-level 40 is only about a 15% CP difference.
The question then is should you? Please note: We are comparing level 30 to level 40 under the presumption said Pokemon is maximized for the respective player level.
The short answer? Most likely no.
It does not have an impact in gym battles
With the current combat system, small CP differences hardly matter; actually, fairly large CP differences do not either. A Dragonite at 2400CP can easily destroy a Dragonite at 3,000CP. The argument that a 15% increase in CP makes a difference in combat is highly unlikely.
Having more of said Pokemon is better for Gym security
In the cases of Snorlax, Lapras, and Dragonite, it makes more sense having numerous Pokemon rather than wasting candies simply on one. I would rather have 3 Dragonites at 2,500CP than 1 Dragonite at 3,100CP.
The amount of stardust is disproportionate to the pros
Approximately 175,000 stardusts is unwarranted. The amount of stardust could be used boosting other Pokemon, especially when encountering a higher-CP Pokemon. The stardust is better utilized elsewhere, like a Pokemon captured at a lower CP, but possessing a high IV (in some circumstances).
Higher-CP Pokemon last longer in gyms
It is not unknown that competition is getting stiff. Some people battle their own team more than other teams. With more and more players reaching level 30, gym Pokemon’s cp are naturally increasing. The lower-CP Pokemon are the first eliminated; perhaps the extra CP will allow you lasting long enough to capture your daily gold an extra day. As Billy Burrows, a Pokemon Trainer via Facebook, points out:
“Buffalo NY is a warzone, you need to constantly be grinding for high iv and good move sets When you find your good defenders power them up and get them in a gym and never look back. When they get kicked out of a gym and come crawling back hit then with the max revive, power them up more and get them back in a gym…”
Some argue the psychological impact of having a higher-CP Pokemon (such as Dragonite at 3,000CP) could deter attackers. Whether this is a scare tactic or a time one is up to the person presenting the argument. Still, in the circumstances of lone players or power levelers, this argument may hold a certain truth.
High-CP collector and/or the casual player
Some people simply want as many high CP Pokemon as possible. In their view, it is better investing everything in fewer Pokemon because they maintain a lower amount of gyms (casual players). As an example, plenty of Eevees produce Vaporeans which act as buffers for medium gyms; in this perspective, it is better having three powerful Pokemon if only wishing to maintain 3 gyms rather than an army of lower-based CP Pokemon that never are utilized.