>>52140572You could go for shinies, but you can also go for good PvP teams to get special rewards through PvP, completing the Pokedex is not as easy as you would think, since buying/trading or breeding traded mons only gives you a blue check in the dex, and green check only comes with OT mons. Some random Pokemon are actually hard to find, which makes it somewhat fun.
Personally I've completed all the regions, have never bothered to go for Second Round of E4s however. Now I just go EV train some mons and do some PvP here and there. Randoms is fun, like in Shoddy battle or whatever the fuck that was.
Tl;dr, it's definitely worth it if you're mature enough to understand the concept of a MMORPG. What you do in post-game it's up to you.