I think it has a lot to do with space limitations. Battery would lose about 1/2 of it's capacity for the extra room required for clips, compartment and the battery's own plastic case. Either that or the X5 would need to be larger. It's a performance compromise and one I that prefer in a performance based flagship product that needs a bit of current to do it's thing. 2 replaceable batteries for the same playtime as the one included wouldn't thrill me. Making it larger to hold a larger replaceable is also something I'm glad that they chose not to do.
Edited by goodvibes - 11/13/13 at 8:13am