I guess one way to look at the question posted is to break it down into two questions:

1) What kind of headache would a person suffer by converting their proprietary RAWs to DNG and then deleting the old proprietary RAWs? and

2) Can we do something to mitigate such a headache?

I'll elaborate on both below... and my believe is that in the end, DNG can be worth it. As long as you cover all your bets. I won't go into the true benefits of DNG because that will take time to manifest itself but its inherent benefits are already manifesting itself. That can be amply learned by googling.

Here goes...

DNG being a late-comer will suffer from being less well-known by some of the photography software tool makers out there.

For example you may have decided to make use of DxO Optics Pro or DxO Film Pack only to discover it expects you to load up Nikon NEF raws or the Canon CR2 raws. But it won't take DNG. Yikes!

So this is currently some incentive to hold on to your proprietary RAWs. If you love Lightroom then this is a reason to import you raws as is into your Lightroom Catalogs unchanged. Lightroom offers another possibility--"pregnant DNGs": importation where your "proprietary" raws are embedded bit for bit into DNG and then the DNG is imported into your Catalog. That's what I do and I recommend it. These pregnant DNGs are about twice the size of your regular DNGs.

There will probably come a day when every software tool maker use DNGs, when that day comes you can then strip out the original embedded proprietary RAW 1s and 0s out of these "pregnant DNGs" using Adobe's DNG tool.

My workflow is as follows (the data aspect):

Step 0?. Typically I place jobs requiring something like DxO tools (rescues files from luminance and chroma noise pretty well, at times better than LR2, lens distortion correction which LR2 is missing) or Canon Digital Photo Professional (bakes JPEGs nicer than Lightroom from time to time, has lens distortion correction which LR2 is missing) in a work flow stage BEFORE importing into Lightroom. These are typically batch jobs.

If they don't need this extra step, go to Step 1.

Step 1. Import CR2s and NEFs as pregnant DNGs into LR2 Catalogs.

Step 2. Edit files in LR2

Step 2a. If I need to call up PS, or some plug-in and utility, pass along TIFFs.

Step 2b. If I need to call up DxO Optics Pro or Canon Digital Photo Professional or something like that, I go and find the "pregnant DNGs" and use Adobe DNG tool to extract the original proprietary raws.

Step 3. Bake into JPEGs.

Step 4. Preservation and conservation: backup the Catalogs with the accompanying pregnant DNGs.

So.. hold on to your proprietary raws by embedding them into DNGs and just use the "pregnant DNGs":

Benefits:

1. You can always access your original proprietary RAWs.

2. There's ONE copy of the image, if you have both a proprietary RAW and a DNG sitting on the hard drive you'll start to ask yourself questions like, which version is better, which one is registered with Lightroom catalogs, and which should I delete. So that's one extra decision making exercise that both takes up time and may create room for human errors (I am sure you are perfect, but no one's perfect after been up 72 hours straight)

Cons:

1. Pregnant DNGs contain both the proprietary RAW and the DNG, that will double your storage. This isn't so bad as long as you have a good editing sense and know which files to keep. But many beginner photogs (I was there once) will have a hard time deleting any file and having your file being double the size exacerbates the storage problem further.

2. One day your fingers will itch and you'll want to batch process allll of your pregnant DNGs to delete the embedded proprietary RAWs--maybe a few years from now. DON'T. What if that future Adobe DNG tool has a subtle bug and ruins some of your files. It's easy to miss such a software error since no one has time to go and revisit those thousands of photos. In that spirit, just recognize that the waste hard drive will be wasted forever. But yes, if you want to, Adobe DNG tool can un'pregnant' the DNG.

The main advantage I've heard is that it's an open format that should always be compatible with whatever software you use (even though it's made by Adobe, they've opened the specification). So, in case Canon ceases to exists at some point in the future, if all your files are in DNG, you would still be able to access them.

I've tried converting, buying into this line about compatibility, but I noticed the DNG images didn't look quite as good as the original RAW files. Mostly it was color issues, they just didn't seem to match up quite right, so I went back to the original files and got rid of the DNGs. I think there is already a good enough understanding of the Canon and Nikon formats that, even if the companies were to close down (which is unlikely in and of itself) there would still be plenty of third-party tools that would be able to read them.