Major corporate houses choose open source partly due to the economic gains and mainly due to the control the open source system gives to their hands.
The quest for control is a serious problem in open source arena. The problem sometimes seems trivial, but is really a loss of manpower by the professionals developing programs in open source environment. The problem is seeking too much of control leads developers develop their own solutions for specific issues.
Reinventing the wheel is quite a common problem in open source arena. Instead of using an already popular script, developers tend to write there own scripts for specific tasks. There may be hundreds of scripts with similar functions. There are however instances when a developer has to come up with something new when the existing system doesn't give the expected results. Developing a code from scratch will be faster than modifying existing program.
Proprietary software, in this sense is quite advantageous here. The developers working with proprietary software hardly ever goes to coming up with their own script as a solution for immediate problems at hand. Then again, proprietary software developers too invent their own wheels, but the goal is to avoid nasty situations like legal problems.
The nature of shared development of software development appeals to the human nature to have control over their destiny. This is true even when they can get customized 'solutions' by proprietary software vendors. The future is in open source, but it is premature to make predictions as to when Linux or an open source OS cross runs on 50% of the computers of the world.
Incidentally Microsoft Corporation's Shared Source Program is an effort by the corporate giant to somehow win the trust of open source community. Though Microsoft allows a peek into the source code, redistribution rights are neither granted nor expected. Only an illusion of free handling of source code, shared source program lacks the democratic nature of all open source initiatives.
Back to open source initiatives - open source initiatives find it quite difficult to get professionally talented developers to the fold, especially in the beginning stages. This is true for almost all open source initiative. The situation reverses when there are enough people in the fold and the project takes momentum at SourceForge.net.
There is one last thought for major corporations that have become so stepping on open source shoulders – spare some of your resources to enrich free software and open source software community.