I know it flies in the face of web rationalisation; but the only way to get better at building websites is to build websites.

I'm unquestionably better at this job than I was when I started a couple of years ago. I learn something with every new project, and every fresh set of client requirements. I always consciously try to add something new and innovative - for me, or for government, or for WordPress - into every build. If it works, I'll do it again next time; if not, I'll certainly be wiser for trying.

If you adhere to the 'only one website per government department' rule, that would mean your team is only building one website every 3-5 years, or maybe even more. No opportunity to practice or experiment in between monster projects; and the experience of 'last time' will, in all likelihood, be irrelevant. Any mistakes you make, you'll be stuck with for the v-e-r-y long term.