A vac pan will be no greater risk of being a breeding ground for insects than the rest of the central vac system. I don't understand why your husband would think that food would stay in there, unless he's thinking that he can just sweep wet cheerios in there. If the intention is that wet foods can be swept in there, then yes, it would get stuck to the sides of the pipes. Never sweep wet stuff into the system, but isn't that the same of all vacuum systems other than WetVacs? The suction created throughout the system is the same in the rest of the house.

We have one in the kitchen and it is nice unless you have a DH that will sweep everything up to that point and then is too lazy to bend down and open the vacpan. We have had it for 6 years and I have not noticed any critters around there.