The Shard won't always be 'lower than the sun'. If you are standing beneath it, it will almost always be higher than the sun, and for everyone that can see it, it will be higher than the sun when the sun is just rising or setting. Certainly it is true that there will be conditions when the sun is high in the sky and would not shine into windows much when it could reflect into windows with a pretty high power level. Consider it to be a big mirror and it is evident that its reflections can go almost anywhere. I'm sure some north facing rooms that never see the sun will appreciate some sunlight shining in off the Shard.