It seems that because the hydraulic fluid is shared between the brakes and clutch something seems to contribute to contamination.

 

It's moisture.

Even in relatively dry climates, brake fluid will absorb moisture from the air through the hoses and every time you open the cap. As little as 3% moisture will drop the boiling point of a DOT 4 fluid from 400degF+ to as little as 311.

Having said that, Lotus is at the extreme end of the scale when it comes to recommending replacement. Most U.S. manufacturers have no replacement interval, while some EU brands (ex. BMW) specify replacement every 2-3 years. For cars that see track use or other extreme braking situations often, it's cheap insurance against brake problems when you least want them.

 

Having said that, Lotus is at the extreme end of the scale when it comes to recommending replacement. Most U.S. manufacturers have no replacement interval, while some EU brands (ex. BMW) specify replacement every 2-3 years.

As already pointed out the brake and clutch system shares the fluid reservoir on the Elise. I don't know what it is - probably the much larger movement of the seals in a clutch system - but the fluid in the clutch systems of every car seems to get "dirty" much quicker than fluid in the brake system. With the fluid being shared, the clutch fluid "contaminates" the brake fluid rapidly. I'd go ahead and flush out the old fluid yearly in the Lotus.