Dexterity was supposed to improve your movement at a high level. As you said, the rolling becoming a backflip was proven to be the sole effect of a certain ring. I have been asking your question on these forums but no convincing answer.
Some say DEX increases your rolling speed itself as well as the distance you roll. Hasn't been verified.
Some people say they lvl 10 warrior with 13 dex moves just as fast as their thief with 30 dex...
When it comes to STR, NO ONE has been able to provide me with clear cut evidence of any improvement regarding the attack speed.
More importantly, the official guide makes no mention at all of those two facts.
In conclusion, I do not believe those two stats to have any secondary effect at the moment apart from scaling with appropriate weapons.
And that's a shame.
Of course, I would love to be proven wrong so please guys, do so.
The dark souls wiki is indeed not very accurate at the moment.
It still states that there MIGHT be the 'second chance' spell in this game and that the pre requisite would be faith 20. It has been proven that it is NOT in this game. The wiki isn't a valid source at the moment.
As I said... the official (and excellent) guide by Future Press makes no mention at all of those two stats increasing movement rolling speed or swinging speed.
It is a shame in the sense that i believe this would have really improved the variety of builds going around.
Oh!! Been strength secondary effect has yet to be 100% proven? That's a shame ;_; now i regret pumping so much into dexterity when i can just turn the weapons into non scaling types and focus on vital. I see a lot of pvp people do that. --- Future Lalafell Lancer~ Boku wa Imouto ni Koi wo Suru
The only people who have answered me both on english and japanese forums have told me of no noticeable difference in swinging speed with STR.
I wonder where the wiki picked that up honestly, I was really hoping for a significant change for some weapons like the Hallberd etc. Their move set is so terrible it got me hoping for the rumour to be true.
So yes, both the secondry effect of DEX and STR remain to be proven. So far, judging from the info and testimonies from other players, the answer is no.