Some abilities attack with your offhand and some with your main. The delay only matters in most cases because slower weapons have a higher top end, while faster have a lower top end for the overall DPS. Generally, when tanking as a rogue you want a slow weapon in your offhand. I think this is also true for melee dps, but MM is better anyway IMO. As far as pvp, look at your abilities you commonly use. For instance, you're not going to be using a whole lot of AOE attacks, but you might get parried or dodged a lot so having a fast main hand for more throughput and a slow offhand for unavoidable cool downs is a good idea. I'm not exactly a mechanic expert for combat in this game, but generally this is the idea.