Actually there is a way to have lightweight desktop client with ActiveSync.. its a workaround. you will need a modern mobileOS simulator to achieve this.

The simplest for our purpose is 'Firefox mobile OS Simulator'. It runs as add-on for Firefox browser, this lightweight simulator has an ActiveSync Client built in for emails.

Have put step-by-step instructions here

1. Open firefox browser, browse to https://addons.mozilla.org/‎
2. Search for 'Firefox OS Simulator'
   
3. Click "Add to Firefox".
4. Once the add-on has downloaded you will be prompted to install it: click "Install Now". ( its a big download about 50 MB)
5. The simulator Dashboard opens automatically after installation - you can reopen it at any time by going to the "Firefox" menu (or the "Tools" menu on OS X and Linux), then "Web Developer", then "Firefox OS Simulator"
   
6. To start the simulator -- Click the button labeled "Stopped" on the left-hand side of the Dashboard, the Simulator will boot to the Home screen and you'll need to navigate to your app
   
7. On the mobile simulator swipe/scroll Right to find the E-Mail app.
8. In E-Mail settings click ' manual setup' fill-in the user and exchange server details.
9. Voila you have the Active-sync client running on your desktop.
10. for help with any issue refer - https://developer.mozilla.org/en-US/docs/Tools/Firefox_OS_Simulator

This is working great for me :)