It's not a 30kg penalty per se: it's 30kg less ballast available. Cars are underweight as it is, and with KERS, (most) cars still won't be overweight (though Webber, for example, would have just 5kg of ballast left).
The problem with KERS is how it was introduced: Just ahead of a major crisis, teams had to spend dozens of millions in order to get their systems functional and reliable. At the same time, the FIA introduced it in a bland, castrated form: With these severe restrictions, a fully-optimized system (and the current systems are far from optimized) is reckoned to be worth 3 tenths at best - and currently, the easier-to-setup and better-balanced KERS-less teams are better off staying like this. With the testing-ban in place, it'll be hard to make progress with the system, and most of the KERS-teams will focus on a double-decker diffuser.
The plan was for 2009 to be an introduction year, and then to slowly crank KERS power up in the coming years - for 2010, the plan was to make it obligatory, and increase power by 50%, as well as the time it is available. However, politics and cost-saving have gotten in the way, and some teams (Ferrari mainly) are currently lobbying for a spec-KERS system from 2010 onwards - effectively killing the whole idea of KERS as a competitive element, and leaving it a "green-cred" marketing tool only. I pray this doesn't happen, and I hope KERS gets more power next year (this year is lost - rule-changes will need unanimous agreement, and I want to see the guy who persuades Brawn, Toyota and Force India that other teams are entitled to get an advantage).