It's not checking one extra Pokemon, Hydreigon could always check Bisharp. It's about checking an entire playstyle, and an entire new role for Hydreigon, this of a hazard remover. Hydreigon is the hardest Defog user to wall, with similar power to Latios but without the awful Pursuit and Aegislash weakness, which is just excellent, and a way more spammable STAB in Dark Pulse. DeoSharp + Aegislash are all fucked up by Hydreigon with a moveset of Dark Pulse / Defog / Superpower or Fire Blast, which is not something many Pokemon can claim. Defog Hydreigon would be the most reliable offensive hazard remover, as it's not weak to Pursuit users or Bisharp. It would also be able to simultaneously check Aegislash and Bisharp, something no other Defog user can do without running defensive sets that are easy to wall (Zapdos and Mega Scizor).
Dragon / Poison Haxorus would become an even better wallbreaker thanks to its ability to OHKO most Fairy-types with Poison Jab. Also, it would gain more set up chances against Pokemon such as Specs Keldeo locked into Sacred Sword, Mega Venusaur, Terrakion's choiced locked Close Combat, and Toxic users without the need to use Lum Berry. The resistance to Mach Punch is nice too and would make Haxorus a tad harder to revenge kill (the Dragon Dance set, the other sets are easy to outspeed with most offensive Pokemon anyway).
Slack Off Mega Blastoise would be great for the the reasons that all the people so far have mentioned. Right now, i am torn between Defog Hydreigon and Slack Off Mega Blastoise, bu i think i will go with Defog Hydreigon, as by becoming a better Pokemon we gain one more viable check to huge threats, namely Bisharp and Aegislash.