I have met many companies recently who not only have outsourced their operations to a 3PL but they have also turned over the procurement and carrier relations functions as well. I think this is a bad idea.
I believe this for at least three reasons. First, and the most obvious, is you have turned over the entire budget to a company which, most likely, has conflicting interests to your own. At some level, the 3PL is interested in making money for their company and many times actions which accomplish this do not also help the client company. Can you develop complex gainshare algorithms which limit this problem? Yes, but it is very unlikely you will get them to work.
Second, you limit your ability to exit the 3PL relationship if needed. Everyone goes into 3PL relationships thinking they will never end and this makes sense. You do not get married and immediately plan on getting a divorce. However, in business, ensuring you have an exit strategy is a good and prudent thing to do. When you turn over the procurement portion to the 3PL you have complicated any exit if needed.
Finally, you will lose critical intellectual capital. When dealing with a 3PL it is important the shipper maintain the intellectual capital needed to fully understand the areas of warehousing and transportation management.
For all these reasons I would highly recommend shippers retain the procurement function. Just seems to make sense to me.