It's fair to say that not all vfx facilities produce the same quality of work. That's a function of many things, but primarily if the facility can charge a higher rate they can afford to employ more experienced staff and put more of them on a project. Same with any business.
However not everyone wants to pay what ILM or Weta are charging.
On top of this, the technical limitations of GIMP and Blender are much more limiting than what high-end apps like Nuke/Mari for texture painting (want to do 32k x 32k texture painting at full 32-bit float for RGBA), and things like XSI rending to PRMan or Arnold, kicking around 200GB of geometry at render time.
GIMP and Blender can't get anywhere near those use-cases - yes, they're extreme, but those are the things companies like ILM, SPI, Weta, Framestore, etc deal with on a daily basis.
My theory is that instability in the industry caused by the studios is what's leading to very uneven work. Some of these studios will pay top dollar for big name actors and directors but are willing to have a bunch of random VFX companies from all over the world try to work together to build a coherent vision.
This could end up with Hollywood shooting themselves in the foot. Movies are getting more and more CGI, but CGI that breaks the suspension of disbelief can really ruin a movie.
A lot of people complained about the VFX in Wolverine, for instance, but shouldn't the VFX in that have been better than the X-men triology? Well, when you have seventeen different companies doing VFX shots for a movie, you're going to get uneven results. There was some really good VFX in Wolverine, but there was some really bad stuff too, and I'd have to believe the studio going cheap is what led to it.