can high speed memory interfaces like GDDR5 or XDR ever become mainstream?
GPUs are often already on a plug in module. These modules often have limited room and/or component height allowances. So DIMMs are typically not really feasible from a mechanical standpoint.
DIMMs place a rigor on fixed pin assignment and order. This can add significantly to the overall trace length when the routing a memory bus through a DIMM connector. 
Each connector in a PCB interconnection scheme represents at least some kind of impedance discontinuity to signals that route through the connector. This can induce reflections which impact signal integrity and jitter.