My understanding is that WEI is measuring the overall throughput of the system as a whole, and not merely clock speed of the installed DIMMS. So running faster RAM on a slower Front Side Bus won't actually help you much if your bottleneck isn't the actual memory modules. By way of example - Some of you guys are getting lower scores on DDR2 1066 stuff on a 1066 FSB than I am running my RAM at DDR2 800. How can that be, you ask?? I have my Front Side Bus cranked up to 1600 Mhz.
The general consensus is still correct, tho: It's just a general rating. Not some kind of benchmarking tool.
Best (blog) explanation in English I've found in WEI:
Windows Vista Team Blog : Windows Experience Index: An In-Depth Look