The correct translation given by Duo - "He reads, whereas I talk." doesn't make sense. Whereas could be replaced with "but" so it would be "He reads, but I talk" which I don't think was the intent. Alors que can also be translated as "while" or "when" (I think) which makes much more sense - "He is reading when (or while) I am talking."
"Alors que" means "whereas" although it is sometimes translated as "while" this is only correct in cases where "while" implies "whereas"
If what is intended is simply that two things occur at the same time then "alors que" can not be used - it should be "pendant que". Crucially "alors que" is used when there is oppostion between the things occuring and where that opposition is important for the meaning of the sentence.
So for example in the sentence "he reads while I read" we could not use "alors que" because obviously there is no opposition - instead we would use "pendant que".
Also if there are two different things happening but the speaker simply wants to say they happen at the same time then again it is "pendant que". So when the French sentence uses "alors que" it has a particular meaning that can be missed by translating it as "while" because in English "while" is ambiguous.
So I would suggest that this distinction is not just for the purists.
Another perfectly acceptable explanation can be, "He is reading, even though I'm speaking". This clearly shows opposition in a more easily understandable construction for English speakers. It can't always translate this way, but in this instance it works (as of 16/12/2014).