If Yousuf had used a spinner, then they would have stayed and played some more. Being the "smart" captain that he is, he persisted with a quickie, and as a result they walk off when Pak could have finished the tail tonite.
Yousuf did right thing. Spinner are getting nothing from the wicket they would have done nothing but to ease the pressure and some (20/25) free runs in remaining overs.
Equation is clear. Pakistan should get them out (i.e use seamers) else take the light.