There's good arguments both ways.
It really comes down to a lot of variables based off what is needed, what resources the agency has, and what their purchasing department wants.
I could have certainly done an request for proposal, but I'd be totally reliant on the vendor to actually put the effort into designing the system. Past experience tells me that they'll often low ball the bid to get in the door, then change order the bejebus out of us.
On the other hand, I pay up front for a consultant. We work together to figure out the needs, figure out how to meet those needs, and then design the system ourselves. The consultant knows the industry and the technology. They have the propagation analysis tools. I know the local agencies I can get tower space from. I know the infrastructure we'll need to backhaul. Since this is a large enough project, the towers/sites that will be built will be mine, not someone else's and absolutely not the property of the vendor where I'm having to lease them back.
I'm also not tied to any one vendor. I'm not tied to the 'installer du jour' chosen by the vendor.
And, if the consultant and I screw something up, we fix it. If the vendor screws it up, then legal gets involved and 5 years after I retire we might actually get a functional radio system.
I guess it comes down to how much trust you want to put in the vendor. I've been burned, don't want to get burned again. Remember, those guys are looking at shareholder profits, not providing the best system possible.