Not convincing me at all. You argue like Gengar is some kind of tank, while it's not. Obvsly Jellicent is more useful in this role, but I believe you wont put Jellicent with Deoxys-D you'll chose Gengar, and it will spin block. Get a scarf set with Destiny I'm sure your hazard wont get spinned away. Moreover you will can put an insane offensive pressure than Jellicent.. well Jellicent is just a totally different deal. It's like comparing Scolipede and Forretress as spiker or something like that.. Then you list threats, and well I know for each of these threat a way to shut them down with the right Gengar build. Juge a pokemon potential on what you usually see/use is seriously a mistake.