VMware puts more and more functionality only in the Web client, therefore everybody is 'forced' to use it. I mention 'forced' because many people don't really like the behavior, speed and usability too much - not to mention Flash.
Anyway the web client is getting better and the new functionality convince even the last Windows vSphere client fan.
VMware released a helpful KB article to optimize the Web client performance:
Use the Adobe Flash Player Website Storage Settings to grant Adobe Flash Player access to unlimited disk space for the vSphere Web Client instances.
- In a Web browser, go to the Adobe Flash Player Website Storage Settings panel at http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html.
- Select the URL for the Web Client from the Visited Websites list.
- Use the slider to set the Flash Player storage setting to Unlimited.
You can also right click on the website, select settings: