I agree that it shouldn't be made mandatory, but here's my thoughts on how it helped me:
Firstly, spending time out of uniform gives you a slightly more external perspective on the organisation and the way it works. I think this is helpful because it's important to know how new cadets, new staff, and the general public will see us. Once you're firmly rooted in the rituals and habits of a particular organisation, it's less obvious how 'outsiders' will view you.
Secondly, as I was ageing out as a cadet, becoming uniformed staff seemed like a brilliant idea. For me, it seemed like the 'automatic' thing to do (a bit like university). I'm starting my VR(T) application now after spending two years as a CI, and I think that I have a better appreciation of what is involved and expected (I took on a lot of responsibility as a CWO and thought I had a good idea of what it would be like to be staff, but in hindsight, I didn't). A commission will be worth more to me because of this, and the experience I've gained will definitely help me do a better job.
These thoughts apply only to my personal circumstances, and I know loads of people who have gone straight into uniform and managed perfectly well. I think that each approach has its own strengths.
Off-topic: Something I do think should be made mandatory (maybe only for uniformed staff) is to spend a period of time at another unit. Everyone I know who's tried it (including me) has found it helpful, even if they didn't enjoy every minute of it.