I'm far from being a technical specialist in this area, but the last question you ask is something I have been through and can relate to the empirical aspects of that experience.
I believe DDC integration provides more convenience and reliability, which in turn perhaps may yield superior accuracy of colour and tonal rendition. Without DDC there are several steps in the calibration process you need to implement using the controls on your monitor. With DDC these steps are performed automatically and mathematically. You don't need to touch the monitor controls. I'm sure there is more to it than this - Jack or others who know it better may wish to complement and/or correct these remarks
Up to this past August I used a Dell P992 (Dell's branded Sony Trinitron) 19 inch CRT - a VGA device. Neither the video card nor the monitor were DDC-compliant. Nonetheless I calibrated it and profiled it using ColorEyes Display w. Monaco Optix XR and it provided me with a very successful colour-managed workflow.
Like it was predicted to happen, after four years of fairly intensive use the CRT started to die, and there were no high quality CRT options to replace it with. After considerable research into cost-effective replacement options and much consultation, I bought a LaCie 321 which is DDC compliant.
The main thing I had to do to replicate the previous reliability of my colour-managed work flow was to select a much lower luminance setting than the monitor is capable of. While the monitor is capable of 250, my setting is 120. I print on Epson Enhanced Matte, which is a low reflectance medium, and I find the 120 luminance setting setting provides a reasonably reliable soft-proof of the impression I will get viewing the print. That is what this whole business is all about - whether the viewing impression from the monitor and the print are close enough for your needs - which would vary from one purpose of photography to another.
As the tehcnology advances and options multiply, it gives us more means to do better and better things, but the side effect is that it gets more complicated. I believe that compatibility has become more of an issue as a result. This puts more of an onus on ourselves to better define our needs and research the necessary conditions for delivering them, which includes considerations of product features, cost and compatibility factors. As consumers we're in the fortunate position of just needing to know how to chose. The providers have a much more daunting task looking at the sea of complementary or non-complementary stuff they need to contend with and working out how to design their own products in ways that keep us happy.
A self-interested New Year's wish from me to them is that they have great success in doing so - meanwhile forums like this will always be great for asking all the good questions and hopefully getting some useful answers!