You don't understand correctly.  The table data is sent by reference; the table load SAMP message just contains a URL pointing to the serialized table.  So it's not XML within XML, and you can send any resource you like without having to ASCII-ise it.  For exchanging images of course we use FITS.  In fact if you look at the list of MTypes at http://wiki.ivoa.net/twiki/bin/view/IVOA/SampMTypes you'll see that `table.load.fits` and `table.load.cdf` are in known use alongside `table.load.votable`.  The SAMP standard neither forbids nor makes it difficult to use those, and you're free to do that if you want to, or indeed to make up your own MType using a table encoding format of your own choice.  But if you do it, you'll find that you will be unable to exchange tables with most (though maybe not all) of the existing community of SAMP-aware applications.  Avoiding this fragmentation of the application base is why it's considered good practice for SAMP data exchange to use one conventionally agreed format, which for tables is VOTable, so use of the fits and cdf variants are discouraged except for special purposes.