There's been discussions on the Steam forums for a while regarding this topic (the most recent one, the one that drove me to post here, being here: http://steamcommunity.com/app/435150/discussions/0/1519260397781897808/?ctp=2).

Some people also argue that spears should have access to Scoundrel, but that's not what I'm going for, and not what my points will be based around.

The main issue is that non-dagger dual-wielding is invalidated by the fact that daggers have their own skill tree, thus giving them far more options and versatility, while also having access to the Warfare tree. Additionally, dual axes, swords, etc, are simply not as viable as two-handed weapons when limited only to the Warfare tree, due to the damage difference.

Stealing some quick points from someone else in the thread, while also reiterating my own:

11/15 Scoundrel skills require daggers
7/15 Warfare skills require a melee weapon (including daggers)

This means daggers have access to 18 weapon skills vs any other melee weapon.

Additionally, allowing more weapons to use Scoundrel would add far more versatility in builds which is, in my opinion, something sorely lacking in DOS2, despite the major improvements it has over its predecessor. With the limited number of useful talents (which we lost more than gained since the EA, let alone DOS1 which had infinitely more options, but I've already made a thread about that in the past, so I won't go into it again), DOS2 is already limited in build options.

Scoundrel's technique damage multipliers are based on people consistently landing backstabs, meaning that the multipliers are almost equal to Warfare multipliers, if not slightly lower due to daggers having slightly more damage than other one-handed weapons, post-backstab (pre-crit multipliers). This means that, at worst, allowing more weapons to use Scoundrel, even if only limited to potentially dual-wielded weapons (and perhaps spears, albeit still not something I'm going into), Scoundrel techniques would, on average, still deal less than their Warfare counterparts, meaning no build is suddenly going to spike in DPS.

TL;DR - remove the backstab component of Scoundrel techniques for non-daggers, allow one-handed weapons (minus wands, obviously) to be used, even if only a portion of Scoundrel techniques. A lot of dagger builds are using Warfare, as Warfare techniques can backstab (90% of them, anyway), so allowing the opposite to happen will only promote build diversity. Due to the consistency of dagger criticals, Scoundrel's crit multipliers will still apply to daggers more than, say, axes or swords, but the damage will still be roughly similar.

PS. I'm currently using a mod that allows any weapon to use Scoundrel (albeit only sticking to dual axes at the moment), and I can safely say that both my twohanded and dagger build are still out-DPSing me, but it's certainly fun to have more options.