I do not doubt Richard's results, but question this part:
"If it means that no water gets into the fluid, then the extra cost of the DOT 5 is offset by the elimination of changing the brake fluid so often."
I do not expect that DOT5 does much to prevent water from getting into the system in the first place, just that water acts differently once it gets there. Braking systems are vented to the atmosphere, and rubber hoses are somewhat permeable. My understanding is that the concern with DOT5 is that while the water that does enter the system will not dissolve into the fluid, it will instead settle in the low points, and cause corrosion or vaporization problems.
For this reason, I would speculate that DOT5 would require more frequent flushing, not less. Perhaps not the entire system, but enough to purge whatever water has settled into the calipers.
1986 300E 5-Speed 240k mi.