I agree completely with your conclusion....Apple needs to support AVCHD/MTS/m2ts natively to avoid the very lengthly conversion/import/export times. I was planning to download the trial version of Vegas to see how it handles AVCHD but based on the software that came with my camera the import from the camera is simply a disk copy from flash memory and does not involve a conversion. If Vegas can manage and edit these files natively then that is a significant advantage. I don't know the technical or licencing issues behind this but given the the importance of AVCHD now and in the future unless they can do something about this then Vegas looks like a very good replacement for iMovie and I'm disappointed to say that...was planning to buy a new 24" Intel iMac but maybe now its not such a good idea.
The greatest benefit of AVCHD and the reason it was developed was to create reasonable file sizes for HDD and Flash cameras...those same benefits apply to the storage on your PC. Native AVCHD support means no long waits for conversion, no loss of quality or function such a 5.1 surround (which is dropped by iMovie and everything else on the Mac), smaller file sizes and avoiding the need for multiple files (orginal AVCHD, Voltaic conversion to AIC, import to iMovie, export to iPhoto, copy in iPhoto library etc. In the last 24 hours less than 2GB of AVCHD content on my Sony camera has consumed almost 30GB of space on my iMac.