People complain that their favorite app isn't ported to Linux, but when such an app is ported they complain. With the recent release of Nero cd burning software for linux, I've read a few gripes like why Nero used gtk+, is it free (as in gratis)?, why even release it since k3b is so good, etc. Sure it is easy to find something to complain about, but Neros release is competition for open source equivalents, and I believe that most competition is good for open source.
I just installed KDE 3.4 on Gentoo a few days ago. This quality of this software is leaps and bounds above what I got when I first installed Linux in 1998. Why? KDE and Gnome continue to leap frog each other in features, improvements and applications. They push the buttons of developers from the other camp and the software from both sides sees improvements. With only one desktop, this innovation would be a lot slower. (I actually use a lot of gnome software as well, gnomemeeting, gimp, inkscape, but realize that "purists" want equivalents in kde thus improving apps on the kde side of things).
Gentoo is a similar story (I'm not saying the gentoo is the perfect solution for everyone), when I first installed Linux (using rpm-based distros), I would have to re-install every 3-6 months to get the latest software. (Trying to update an install would usually break it). Been using Gentoo for 2 years now, and the days of reinstalling appear to be over (I'm sure Debian people can say similar things). (Yes, I do have to wait for compiles, but with the new breakup of kde packages that time has shrunk significantly). Debian and the rpm based distros didn't provide Daniel Robbins with a system that felt good to him, so he went out and made his own. And I think this competition of new distros has made Redhat (or Fedora Core or insert distro of the week here) better. I'm using FC3 at work and Yum and synaptic are definitely an improvement over straight rpm. An improvement that wouldn't have come along as soon without pressure from distributions such as debian and gentoo.
An added benefit of open source competition is that it allows projects to cross pollinate each other not only at a high feature to feature level, but at a source code level as well. The transparent nature of open source can allow for frameworks and libraries to be used and improved by both camps. Freedesktop.org is an excellent example.
These are two simple example, but I still believe competition is good for open source, even if it comes from commercial outsiders. Consumers love competition (it drives down price and/or tends to improve quality). Instead of berating Nero for releasing a Linux version of their software, let's welcome it and see what we can learn from it.... I'm still waiting for DVD mastering software that is as easy to use as k3b is....