I agree with most of what you are saying, but I definetly think mBTC at least (or mXBT) should be the PRIMARY denomination. The thing I like about re-denomination though is not having to use those unwieldy prefixes. What naturally follows then, is that IF XBT was to be anything but full BTCs, it should be uBTC so that we never have to move again. mBTC may be enough, but I'm not sure. Also, it should not be lower than uBTC so we can still have to two-place precision divisibility.
The whole point of talking about bitcoins in terms of BTC, mBTC, or µBTC is to ease communication, especially in person where it's hard to verbally say .0245 BTC. Our languages and methods of speaking numbers have reached a point where we are most comfortable speaking numbers with three to four digits to the left of the decimal place and two to the right of the decimal place. Most people would understand if I spoke 123.45 as "one hundred and twenty three point four five" or 432.25 as "four thirty two point twenty five". While we are more comfortable with speaking in the thousands than speaking in the thousandths with 54,321 as "fifty four thousand, three hundred and twenty one" compared to 2.1234 as "two and one thousand two hundred thirty four ten-thousandths", we still naturally gravitate towards discarding low decimal places and lowering the number as a whole.
If 1 XBT = 1 µBTC were to be accepted as the standard today, many of the digits would be so insignificant that they would be ignored in verbal discussion. When we see something being sold for 4.25 USD, we'll usually tell our friends "It's four bucks", because cents are pretty worthless nowadays. Verbal discussions would instead use kiloXBT, or some slang derived from that. If my coffee is two dollars, or ~15,000 XBT, I'm pretty likely to pick up the slang and say "It's fifteen kilobits". Talking about larger purchases would be even more extreme (MXBT?).
I guess what I'm saying is that right now, people would be comfortable referring to most amounts of bitcoins in the range between BTC and mBTC. Switching the digital representations to be based on 1 XBT = 1 µBTC would not remove the prefixes, just make people use different ones. If we're going to be using prefixes either way, why take the extra step of re-denominating? Moving around in verbal discussion between different prefixes may take some years for everyone to get used to, but it will be done, regardless of what the base unit is. We shouldn't fully change our means of referring to bitcoins just because some software will require some readjustment.