you don't, you see people mentioning the development cycle, which isn't the same thing.
e.g. IE9 will go final, the dev team will then move onto IE10. we will see nothing for 2 yrs. only patches to IE9.
If you are a company with 1000+ employees, rolling out a major upgrade to a browser is a big task (especially with IE as each version mangles things other browsers can render perfectly), so you have to keep planning these rollouts, then executing them. This means many workplaces are stuck on IE6 at the moment as IE6 was such a disaster that moving away from it requires a LOT of testing/rework.
If you are a developer, IE is evil. If you are a developer and don't agree with this, you should maybe look for alternative employment
It is the same thing, I don't get why you think it's different. IE is on a 1 year release cycle, rtm, it's also not tied into the system as much as IE6 is. The longer cycle you talk about for business is actually something they like, they like schedules that don't force them to rush things, 2 years is their sweet spot, it's also more or less always been Windows release cycle (and IE was tied to windows till IE8). Workplaces are stuck with IE6 because they have inhouse apps built for that that they don't want to change, that's they're own problem and MS has XP Mode that lets you run IE6 in a VM just for those apps which lets you upgrade the host browser whenever you want, no problems at all.
As for the web dev, if you know what you seem to try to indicate you know, then every browser renders some newer things a bit different, Chrome/FF/Opera, they never always match 100%. That aside, with IE9 that's not going to be an issue since MS is pushing ahead with standards.
In the end IE9's 8 week schedule is fast, Chrome was slower till Google decided to play the one up game and move to a 6 week schedule. Really the end user isn't always updating to the newest nightly build of browser X either, that's more for devs to see what changes in the engine have been made. To that extent the IE9 PPs are for devs and thus don't come with any UI to them.
Getting a new beta or RC for IE9 basically every 2 months till RTM sounds good to me.