seriously? must be 5g and a couple of extra ball bearings which is nothing for your average MTBer. Countering that you have increased stiffness allowing thinner walls. Also, a 1.5 straight tube will be cheaper to produce than a tapered version and the extra money could be used for better materials or better engineered dropouts to regain that weight lost…
Positives of a 1.5 straight are increased stiffness, a greater choice of forks and stems, smoother headsets and/or a great range from an angleset.