First of all they are getting the iPhone this year, it's no longer a question. The way I heard it, the refarm is being done in part so that they can support the iPhone, I don't think apple would release a new iPhone just to support T-mobile, though it's possible I'm wrong.
And I think it will be a problem for T-mobile iPhone customers, I'm on T-mobile in NJ. One of the few areas that has really comprehensive T-mobile coverage and refarmed 1900 Mhz HSPA hasn't gotten to me yet. Unless they have the vast majority of their 4G footprint covered in 1900 HSPA, they are going to receive a lot of complaints.
It's not hard to put AWS in there. Heck, it's almost nothing, as it already has AWS and HSPA+, just not the two connected together. It is currently a business decision to not support it to try and pressure T-Mobile into a deal.