I recall reading a blue post stating that they'll be adjusting ambush to be capable of using non-dagger weapons, but obviously normalized so they don't use the entire top end damage like they do with daggers. Once they do this I agree they should give rogues a starting weapon of a sword/axe with a dagger offhand.
Honestly though I wouldn't contribute this so much to bad players. All it really takes it reading the tooltips when you get new abilities to find out which would be more preferred. Granted nothing in sinister strike says specifically that it benefits more from slow weapons, but common sense should tell you that 'An instant strike that causes X damage in addition to your normal weapon damage' would be better with a higher top end damage weapon.