Moisture between the glass...the largest warranty issue in the window industry.  XL-Edge achieves 20-year success rates of 998 in 1,000.  Others who insulate their own glass units may have failure rates as high as 80 in 1,000 over 20 years.  That's important when you have copper that will naturally patina, exterior finishes that will fade slightly over the years, and your valuable time that is affected by needless service calls.  Your getting the very best windows, why not get the best performing insulated glass that rarely has an issue?