To trust built-in metering or to use external pro-exponomete

[AliBaba]

The build in exposure meter measures reflected light (the amount of reflected light depends on the colors of the motive). With an external exposure meter you are able to measure incomming light (which is the "correct" thing to meassure, and doesn't depend on the colors) and you don't have to compensate for dark/light motives.
If you meassure light with your camera and use a greycard (or experience) then it will give pretty much the same result as using an external exposure meter. You can also use the spotmeter to make a virtual histogram which gives you more information then histograms on a DSLR.
Personally I prefer to use a spotmeter instead of an external exposure meter.