Why Don't the Eagles run more double TE formations?

Last night the Birds played the Patriots in a preseason matchup where backup tight end Clay Harbor looked impressive.  Harbor was thrown to six times and was able to haul in all six passes for a total of 30 yards and 2 TDs.  Obviously Brent Celek is a proven tight end and should be starting, but in this case I feel as though the eagles have another starting tight end on their roster in Clay Harbor.  If the eagles ran more of their offensive plays out of double TE formations they could stretch the field with Maclin and Jackson allowing Celek and Harbor to get open against opposing LBs.  Why doesn't Andy Reid see this...