The offhand operates more or less* independently of the primary hand for dual wielding purposes, I believe, Darco. Otherwise, it'd be essential to use an offhand weapon with delay equal to or slightly less than your main hand weapon (unless you have a weapon that's twice your primary hand delay) to avoid losing 50% of your offhand attacks. That's not the case.
(*I've heard it suggested that this is actually slightly more complicated in that the DW check occurs at fixed intervals but causes your weapon to swing at its normal delay during "successful" intervals, but I'm not sure this is correct, and if it is, the consequences for weapon delay preference are real but fairly minor.)
Rys, I'm thinking what you might have heard about the offhand is not that damage is all that matters, but that *ratio* is all the matters. The primary hand gets a damage bonus
that doesn't scale with delay (not for one-handed weapons, that is), meaning that faster weapons perform somewhat better than their ratios would indicate in the primary hand.
Hence the regular admonition that you want to give slight preference to faster (lower delay) weapons in the primary hand, and go entirely for an efficient (high damage:delay ratio) weapon in the offhand.
The biggest complication is delay rounding. Some delay numbers are just "better" than others at a given haste percentage; for example, at 100% haste, a 20 delay weapon and a 19 delay weapon have *exactly* the same delay. See the table Cromis has worked out back home, or check this Steel Warriors post
for a more limited version).
Proc and proc rate can also tip the scales when looking at two fairly similar weapons, and for rogues, of course, backstab complicates the primary hand picture since BS is tied to weapon damage but not to weapon delay.