The later you get into the game (aka Act 2) the more your equipped weapons lose importance.
What are you talking about? If, at any point, you use your weapons to attack physically, or in other words, are not a mage, then your weapons remain critically important and need to be upgraded each level.
Don't invest into dual wield or single handed skill, they're not worth it, max out warfare first, then scoundrel instead.
Especially once you get the big hitter necro skills you will never basic attack ever.
Dual Wield is useful for a specific dodge-oriented build, which usually means taking Parry Master and being a Dwarf, as well as looking for 3%/7% dodge bonuses on gear. If you're willing to commit to that, you can get over a 50% dodge chance.
Single-Handed for Sucker punch is usually done for a build focused more around CC and status effects. Polymorph is a good addition to such a build.
Otherwise, dual-wield and stick with Warfare for raw damage.