I am not saying Pakistan are the weakest side playing spin, but definitely not the strongest. Swann, a world class bowler was lethal against Pakistan in last years tour. It was just that Anderson had such a superb series, that people put Swanns performance as mere ordinary.
Pakistan have an ageing problem against left-arm spinners as Herath showed when we toured Sri Lanka. Mendis is ordinary, we should be able to tackle him. You have to look at Bishoo and even ordinary spinners like Hauritz.
You could point out its due to our disastrous batting, but isnt that the same thing as our batsmen not playing spin as well as the rest?
We have done progressively better in the past year against Vettori, Harris etc, and this Misbah led team looks much better, but dont count out Herath as the major threat in the TESTs.
In spin playing rankings, I would Pakistan behind India, Sri Lanka, South Africa and Australia.