No, you're making your metadata portable. Save the keywords back to the files and it goes with you to the next app (using DNG makes that even easier as there's no dependence on the other app reading from sidecars). Dependence is on metadata, much more flexible than trying to categorise your pictures through folders.
Why not do both? What if metadata is not
portable? What if DNG never gets adopted by any of the major players such as Canon or Nikon and Adobe drops it in 5 years. So far the only players that really seem to care about DNG are Adobe and the smaller camera makers. The major players do not seem to like it at all.
What if someone like Apple or Microsoft desides that they don't want to play nice with Adobe anymore and creates some new file standard and gets Nikon and Canon to sign up. Or Canon or Nikon decide to work together to create a new standard and shut out Adobe and they get Apple and/or Microsoft to make it a native file for the OS?
What if some new company desides that with the advent of video in DSLRs and cameras like RED that they can create a new system of portable RAW for both video and stills and as such the current DNG and metadata formats do not work well. Then what if someone like Apple took this and turned Final Cut and Aperture into a single product and much like the did with iTunes made a Windows version leaving Adobe and DNG in the dust.
What if the Open Source Community decided that they do not really like the influence of Adobe with DNG and create something different where things like camera profiles and lens corrections could be added by the camera into the raw data file and then be open for any program to read that used this open source format.
What if Sony who makes DSLRs and Sensors for other companies, as well as a bunch of software desides to create a new standard not based on what Adobe currently does and gets Nikon to be on board because they make the sensors for Nikon, and then this becomes the industry standard and Adobe drops the current metadata strategy for Lightroom and drops DNG
There are way to many what ifs in digital photography in 2009/2010 to lock yourself into any one system, program, or way of doing things and having an exit strategy is never a bad thing, especially when it only takes a few seconds to put one together. The only I know with any certainty is that 5-10 years from now I will be working with digital camera files differently than I do now. It may very well be Lightroom 5 and DNG or it might be something totally different.