Choosing MH/OH re: Weapon Ratios

[captnamazing]

Level 33 warrior here. When I was lower level, I was using the slow weapon in Mainhand and fast weapon in Offhand. Since then, I've reversed that because of things my guildmates have told me, as well as stuff I have read on the forums (such as Wurmslayer is best in Offhand). I've noticed my threat being better, but I don't really know why.

Today I scored a cheap WS since the patch changed it to Primary only, and a friend of mine (bard, former paladin in 1999) and I got into an argument, him saying that WS should ONLY be used in MH, and me saying the opposite based on what I've come across researching best warrior weapon combos.

Anyway, I would like to hear WHY I've been told that it's better to have the faster weapon in the MH and slower, better ratio in OH. It seems counterintuitive, considering the mechanics of DW.