While I don't disagree that Pakistan as a state has been largely mis managed and leaves much to be desired in terms of civil/human rights and policy...to say there is 'no geographical reason for their existence' is just an uneducated statement. Now you can disagree with the basis of dividing India along religious lines. But not only is that a different matter all together, but that would assume that there existed a cohesive idea of 'india' to divide in the first place, when that is patently wrong. Rather, both India and Pakistan are republics cobbled together from the dozens of individual states that dotted the Sub continent in the wake of the fall of empire. Tho these had historically been joined in various combinations in various empires, there never was a state or rigid nationalistic concept known as 'India'...it was more of a general region with a shared cultural back drop, much as we consider Europe today. This is sure to rustle some Indian jimmies, but Pakistan as a concept is just as legitimate as India or any other man made nation state. That is to say, it is a nation created out of self determination and man made nationalistic concepts.

Going back to the problematic line in your post, this notion that 'Pakistan has no geographic reason for existing' is one of the more mindless talking points of Hindu nationalists. It is touted around as if Pakistan was just arbitrarily drawn on a map with no rhyme or reason. But this can very easily be dismissed without even getting into a lengthy discussion about history. Simply, take a look at this map:

http://upload.wikimedia.org/wikipedia/commons/2/2c/India_religion_map_1909_en.jpg

Ya think it's just a coincidence that Pakistan's initial borders looked the way they do?

TL;DR: There is a geographical reason for Pakistan's existence.