I am greatly against Aegislash in OU, and would vote to keep it banned from ou. Aegislash serves little purpose in balancing the metagame, and causes many more problems than it resolves.
Aegislash is largely overcentralizing and limits creativity, stifling the metagame into using the same mons so they can handle Aegislash. Aegislash has amazing stats with Stance Change and KS, a movepool that allows it to beat a set that would "counter" a different set, and a great typing, allowing it to check/counter many of the tiers top threats and lesser used but still strong pokemon, making them significantly worse in ou (Aegislash has essentially no drawbacks and can be used on any playstyle and almost any team, which makes the game much less competitive). Since Aegislash's counters are really only defined by its set, people often resort to using the same mons to check it. Bisharp and Lopunny are good examples of among the mons that would rise in usage to an unhealthy level due to Aegislash's presence in ou (and often, they still cannot ohko aegi based on investment and/or drops, and can get ohko'd or cripped in return. This results in a metagame with a skew in usage percentages, and essentially, just screwing up the metagame by restricting so many options that become almost unviable or just significantly less effective thanks to aegislash. This is probably the most significant reason why I believe aegislash should stay banned. It restricts creativity so much, and makes the game much less competitive by limiting the metagame so much.
Some people argue that despite Aegislash being overcentralizing, it still serves to balance the metagame. This, however, is not really true. Some of the mons leaning towards the "broken" side in ou include Mega Metagross (which many players believe should be re-tested) and Lando-I. Altaria is another mon that is much stronger than "average" in ou. Aegislash doesn't even help much against Lando-I, and can still be beaten by the other two with coverage moves (although its ridiculous bulk makes it much more difficult to break through). In the end, Aegislash doesn't really solve many issues, and by creating more than it solves, Aegislash has a negative effect on the metagame.
Another way Aegislash severely affects the metagame is in the form of hazard removal. As a ghost type with such amazing bulk, the most common spinner in ou, starmie, has great difficulty in spinning, even if you catch aegi on the switch with an analytic H-Pump. Excadrill can arguably spin on it, but with an air balloon, even excadrill is limited in spinning. With such bulk and mixed offenses, it really becomes very difficult to spin. For defog, the most common defoggers are probably the latis. Both the latis become much worse with aegislash's existence, especially since it gets pursuit and isn't hurt much by the attacks of many mons it can trap. People can resort to using other defoggers such as Mandibuzz, Scizor, or maybe Zapdos, but all of them can be beaten by different sets, such as fast head smash, offensive life orb, crippled by subtoxic, etc. In reality, Aegislash constricts the metagame in an unhealthy manner in regard to hazard removal.
Another effect of Aegislash is the 50/50's it introduces. For the opponent, almost every turn is a 50/50. If they win, they can potentially force aegislash out with their "counter" or manage to defeat it. If they lose, or if the Aegislash user is aggressive and predicts right, they are rewarded by beating the "counter." This topic is a bit more iffy for me so I won't discuss it much, but aegislash does introduce many 50/50's that don't really make the game competitive (why play the game if flipping a coin and then picking an attack will have the same effect?).
Before, people weren't really sure if aegislash was broken because they hadn't played in the xy metagame without aegislash before the ban. After playing without aegislash, and then having it re-introduced in the suspect ladder, the negative effects of aegislash on the metagame have become crystal clear to me. As a mon that chooses its counters based on the set it runs out of a wide variety, and all the previous assets I mentioned, while having essentially no drawbacks to using it, it is clear that aegislash's introduction into the metagame will restrict much more of the metagame than just the "problem mons," making it a poor decision to re-introduce aegislash to ou.