Its so that they don't attract attention. By using mules that look like X5's the average person just expects that BMW is testing X5's.

Its a pretty common practice for automakers...

Take a look at this... Porsche used M5's as its cover while testing the Panamerica's underpinnings... http://www.worldcarfans.com/spyphoto...ype-spy-photos

Or even earlier, porsche used Cayennes to test out Panamerica mules... http://www.worldcarfans.com/spyphoto...era-spy-photos