The real issue for most companies that use significant quantities of product is cost. To that end, you always have to keep the vendor honest by having an alternative to their parts. I know for a fact that if they want your business, they will cut prices to the bone leaving you all the fat. So you have to be able to switch vendors without significant impact on your design implementation. Even if they won't switch for a given product, they have to be able to use the part that has the lowest cost. One place I worked knew this very well and every new socket was competed on price before the board was designed.
Of course this does not apply if your design requires some feature that is not available in a competitor's part, such as a particular high speed serial I/O capability. Some vendors will tell you that this is not a smart way to go since you can't take advantage of "special" features of each part. I have yet to find an application where using a "special" feature will change the implementation so much that it will allow me to use a cheaper part from a different vendor. I'm sure that can happen, but I don't expect it to happen a lot.
So for most cost sensitive apps, a company will do better to compete the FPGA vendors against each other or you will be giving them the fat on your designs.
Rick