I'm supporting Ash Borer on this, with basically the same logic—Thunder Wave makes it really dangerous for a lot of Celebi counters to switch in (Mega Charizard X/Y, Talonflame, Mega Pinsir, etc.), and sometimes you don't have a free mega slot for Venusaur. Keldeo, Rotom-W, and Mega Gyarados are all good reasons why defensive Celebi is viable.
I don't think I would support an Offensive set anymore, however, despite my past support of it. I would actually use Shaymin over Celebi in most cases, because a Psychic typing and a slightly-more-accurate-yet-still-worse-STAB don't do as much favors to Celebi as they hold it back. And in both cases, they're overshadowed by better, more effective special attackers in the tier, including but not limited to Mega Charizard Y, Keldeo, Aegislash, Deoxys-S, Kyurem-B, and so on, all Pokemon with more valuable niches than "It has balanced stats" and "it counters Rotom-W."
LightningLuxray, I know you've tested defensive sets before, but Ash Borer's set is a little more oriented towards crippling opposing Pokemon with Thunder Wave and avoiding Pursuit, making it far more of a defensive
support Pokemon than a defensive
stall Pokemon. It looks like it could allow most bulky offense or balanced teams to easily outspeed and overwhelm typical HO Pokemon. Try putting it on a more balanced team rather than trying to make it a worse Mega Venusaur, and let me know your thoughts afterward.