When it comes to St Paul’s Cathedral there are many protected views which are likely to be a bit of a headache for urban planners when attempting to build a new structure. They need to consider that the public need to see the cathedral from several public viewing spaces. I believe it’s because of these that the Shard is shaped like it is.
The fact you can see this building from pretty much anywhere is one of the reasons I like it so much, and is certainly something I seek out in the skyline when on a rooftop.
It also means that from the top you get to see…um…lots of public spaces that have protected views.
You also get to see more, or course, so I suggest you head up there when in London to take a look at the awesome view for yourself.