We get to see how functional entirely Free systems really are. Maybe you don't need the latest and greatest nVidia drivers to still have a machine that does what you need it to do.
This is actually worth a great deal. I recently did a new install of Debian and was surprised that Flash seemed to be working just fine. "This is Debian, there's no way there could be Flash on here, how is this working?" I asked myself. It was of course Gnash. After intentionally going to some Flash heavy websites I saw that while Gnash is not 100% Flash compatible it comes very close. This surprised me because I last looked at Gnash awhile ago and it was pretty much useless. I still get the proprietary Flash but thanks to Debian I saw that Gnash has come along a great deal. As it progresses I will be happy to use it, especially because doing security updates for Flash is a pain.
And, if that's not good enough, I recently built a computer to run MythTV. The motherboard had ATI graphics, which I assumed would be unusable with Linux so I bought an nVidia add-in card. It was cheap and had a ridiculously loud fan, so I sent it back. Having nothing to lose, I tried the built in ATI graphics. Debian of course does not have the non-free drivers in the main repository, but I got the open source driver working with minimal hassle (might have had to add non-free firmware.) The open source driver works very well, with full XVideo support which is essential for decent MythTV performance. I thought I would need the proprietary driver for that. So I have happily stuck with the open-source driver and have gladly dispelled my notions of all ATI support in Linux being terrible.
So, yes, the promotion of the open-source drivers and the exclusion of non-free firmware plays an important role. It shows people what open source is capable of and it gets more exposure for open source software and speeds its development. It encourages people who run Debian to look for hardware that is easy to use under Debian. This is much more than some dumb ideological move whose only consequence is to make Debian harder to install.