A dollar is 7,700 XBT at the moment. The dust limit is 54 XBT. Therefore XBT covers quite well the micropayment range and the commas make it easier to read then 0.0077 - 0.000054 BTC. Reading mBTC out of XBT is also easy 7,700 XBT = 7.7 mBTC.
For higher sums 123,456,789 XBT is 123.456789 BTC also easy.
I think people generally more used to deal with big numbers than with small fractions.
Of course, commas and decimal points mean different things in different locales, but that's more of an i18n issue than a knock at picking any particular range to use as a valid level.
I believe that if Bitcoin is going to be successful it is going to need to be at the level where talking about a few XBT is similar to talking about a few dollars/yen/whatever. But for talking about real values I don't see it being useful for a long time yet.
I do agree with the idea that big numbers are easier to think about than mall fractions. But like it or not the worth of Bitcoin is compared directly to that of USD and so we're probably looking at appreciation of three orders of magnitude until XBT are used directly.