Again.. talking about assignment without also talking about the license in use is not enough. BSD/MIT/Apache with assignment is a totally different situation than GPL with assignment. Assignment coupled with an automatically "business-friendly" proprietary permissive license is still a level playing field for all contributors. Everyone gets the ability to make a proprietary fork..to take their copy of the ball and go home..any time they want.
It's only when you mix copyleft licensing which strongly implies a co-development model with corporate copyright assignment where things get problematic and inequity arises between contributors. In this case the specific wording of the contributor agreement can matter a lot in terms of weighing the trade-offs...especially if you ever ever want to use the code you are contributing in another project or in another context.