I didn't say Rushdie was your typical Indian (I am aware that not all of them have been treated to either the fatwa or a booker), neither did I say all Indians exelled in English. If you spend time in India and read a book or a newspaper, or watch television, or talk to people in the street, you are likely to find out that Rushdie is by no means a singularity though. Depending on the social class one is born into English may not be the only, but at least one of the languages, learned from the very start, hence the term native speaker does not seem to be out of place. I do not think that a couple of million people who fall into that catgory should be neglected.
Your comparison of India with Germany is not a sound one, English has neither risen to the status of an official language in Germany, nor has Germany had a significant output of authors writing in English.
Rushdie himself has coined the phrase "imaginary homeland" to describe the fact that he feels stuck between the worlds. Quite a few of his compratriots are similarly versed in English though, and they would proudly describe themselves as Indians with English as their mother tongue.
For "literatures" as a plural check "The Empire Writes Back" by said Mr. Rushdie among other sources. Studies of postcolonial literature(s) in particular deal with the phenomenon of a choice of regional literatures (Indian with Rushdie and Roy as prominent examples, Kincaid from a Carribean background etc.), all of which contribute to what traditionally was understood as existent in the singular only.
for possible definitions of dialect and variety, read on to the paragraph headed "Standard and non-standard Dialects" where you will find Indian English listed among the accepted standard dialects.
Be that as it may, the main point I was trying to make is that "being difficult to understand" can hardly be considered a knock-out for a variety to be recognised as such. I mean what would one do with Yorkshire, or Cornwall, or the south of the US?
Edit merkt an dass ich, wegen dringend anstehender Arbeit, so lang an der Antwort verweilen musste dass Graeme den Link bereits gepostet hatte.