I once worked for an agency that had this type of procurement policy. All contracts were Firm Fixed Price (FFP). There was no other option. This policy was a big contributor to the collosal failure of a $6 Million project. The policy forced us to fix the price for contract activities that are extremely difficult to fix. Think about what is required in order to fix the price. We should have a pretty good idea about how many resources we need and how long it will take to reach the objective with the right level of quality and customer satisfaction.
There is a lot of work that lends itself to this contract type. Installing desktop computers, applying patches to servers, performing a Certification and Accreditation are all examples of work in which the scope is fairly defined, the schedule (duration and level of effort) is defined, and quality and satisfaction are easy to manage. This is the type of commodity work that is well suited for FFP contracts. But now switch gears and consider some of the front-end types of activities like business analysis, requirements elicitation and design. This points back to my Chicken or the Egg post. Without sounding to Laurel and Hardy, how long will it take? I won't know until I start the work. How deep are the requirements? I won't know until I know how long it should take.
When we look at and consider a requirement, any requirement, we should consider the full range of contract options that can meet that requirement and then make a selection based on the tolerances of the owner of that requirement.

Consider this spectrum of contract types created by people at Defense Acquisition University's (DAU) Acquisition Community Connection (ACC). On the left side you have Firm Fixed Price and on the right side you have Cost Plus Fixed Fee. Based on things like how certain we are about the estimate, riskiness of the work, and who should own the risk we will hone in on the contract type that fits our requirement. In many instances FFP is the way to go. But look at this chart. Should we assume that DISA is going to be starting any systems development projects? Because if you are initiating that type of work the chart seems inclined towards the T&M side.
Please know that I am not against fixed price contracting. I am however against any type of rule or policy that takes creativity away from the paradigm. If fact I strongly believe that when you are embarking on a systems development type of project that you should have a hybrid contract approach to the work. I believe that Time and Materials components should be used for the business analysis, requirements and design because they are typically difficult to nail down with the necessary quality and satisfation. But once they are, I believe the development (construction), testing and deployment should be fixed in price.