It is heat or cold that affects the shelf life of the fluid. In the automotive world, the emission controls regulations are more strict than in the heavy truck world. There was a lot of paranoia before SCR was introduced to this consumer market as to whether owners would bypass the urea systems, defeat them, etc. Old (stale) fluid won't necessarily hurt the engine, but the engine may not meet the emissions regulations. To satisfy the regulators, manufacturers agreed to put lockouts on the systems, ensuring that the urea system would function for the 100,000 mile limits mandated by the regulators. The flush procedure is driven by that concern. The fluid itself has a published shelf life of at least one year, if the temperature is kept within certain max/min limits. For that reason, bulk dispensors have climate controls built into the tanks. A vehicle like an X5 will see a wide range of environmental conditions during a year, so it is just to be safe. Some charts show fluid life up to 3 years. As more is learned (and real-world experience is gained) the one year rule may be relaxed at some point. Just my opinion on that last count.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue