You can produce work that looks good, as they technically have the features, but the workflow is generally atrocious for open source tools. This means it takes

You can produce work that looks good, as they technically have the features, but the workflow is generally atrocious for open source tools. This means it takes a lot longer to do stuff. 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.