Can people give me some advantages of wider rims. I am running the largest tyres yet (large 2.25's) on 717 rims and have felt them 'rolling off the rim' when cornering hard. I know that I can solve the problem by increasing they're pressure but will a wider rim allow me to run lower pressures in 'safety'. Are there any other benefits. I'm considering a 721.

Yes, this ios what they achieve.

Tyre pressure is? What are the tyres rated as I was running my ZTR 355 too low with some old MK's – these used to be on some 5.1D in the summer. I got away with lower pressures on the wider rims but not with the ZTR's. I had to bring them up to 50 psi and although less grip and a harsher ride stopped the squishy feeling 🙂

You can fit wider tyres and they are stronger. That's it.

The more difficult the conditions, the bigger the tyres you need, the wider the rims should be, the more strength you get. Simples!
It's all about building a 'balanced' bike! 🙂

721s are great rims I love mine.

I'm guessing they're 2.25" Maxxis? So as big as some 2.5"s

You really shouldn't need* 721s to run that size tyres
*though they'll be stronger

I run my 2.25" maxxis on XM719 disc rims, though sometimes feel they're a bit on the limit. The round(ish) profile of advantages don't help

an EN521 (or EN321) should be lighter for the same internal rim width as the 721, but there are lighter rims for the same width

Tyre I'm running is the new Bontrager XR4. Similar sizes to new Maxxis'
not too bothered on weight, but I don't see any point going heavier than the mavic. Any suggestions on other rims?

Wider tyres and heavier. I doubt wider rims add much, if anything to the strength of a wheel.

Am thinking of replacing my front XC717 with the new ZTR "Crest" for the same reason. I run a 2.35 Blue Groove (which I've failed in my attempt at Ghetto tubeless on the Mavic rim).

The Crest is lighter too 8)