My understanding is the proprietary data the 3rd party converter can't use anyway.
Actually for most raw file formats (such as CR2 and NEF) DNG will move most the proprietary metadata into the DNG safely...it just can't be used by ACR/LR. So that data isn't "gone" it's just stored. The camera maker's software COULD be written to read DNG.
Capture One can read DNGs–it can't use the processing engine in ACR/LR but it can use it's own engine for adjusting the DNGs. However, as far as I know, it can only read DNGs from cameras it already supports, so it's not a full DNG SDK implementation.
Personally, I don't toss my original raw files. For use in Lightroom it's useful to keep the original raws and use .xmp sidecar files since that's a much smaller backup. However, if I send raw files to anybody for any purpose, I only send DNG files so I can embed not only the settings but also IPTC metadata into the DNG file. In that case, DNG makes for a much better interchange format. And, DNG is still relatively new (released in 2004) and has gone through 3 major revs (now at DNG v1.3). I would expect that there are a lot more things Thomas Knoll wants to put into DNG (well, ok, I know that for a fact but can't talk about it :~).
And Leica is not the only company to adopt DNG, here's a list from the wiki page
Casio supports DNG in their Exilim PRO EX-F1 and Exilim EX-FH25.
Leica's Digital Modul R for the Leica R8 or Leica R9 and the Leica M8 or Leica M9 natively support the DNG format.
MegaVision E Series Monochrome back.
Panoscan MK-3 digital panoramic camera.
Pentax supports DNG in their K10D, K20D, K200D, K2000, K-7, K-x, K-r and K-5 DSLR cameras.
Ricoh supports DNG in the Ricoh Digital GR, considered a professional compact, and the Ricoh Caplio GX.
Ricoh GXR is the latest, new and solely system of mirrorless interchangeable lens camera unit use also DNG.
Samsung supports DNG in their Pro815 "prosumer" camera and GX-10 and GX-20 DSLR cameras.
Sea&Sea DX‐1G underwater camera.
Seitz Roundshot D3 digital back, used in cameras such as the 6×17.
Silicon Imaging Silicon Imaging Digital Cinema SI-1920HDVR.
Sinar now uses DNG as the raw file standard for their eMotion series of digital backs.
Also note that Adobe has offered DNG to the ISO to be a part of the revised TIFF/EP standard (ISO 12234-2). That's a slow moving process though. TIFF/EP is what most all raw file formats currently are based on including Canon, Nikon, Sony, Panasonic, Olympus, Fuji.
Again, it's useful to talk about the technical issues of DNG separately from the political issues.