Our County is still standardized on IE7. Javascript API != HTML5
However, it can leverage it.
We use a single deployment of an enterprise application on Tomcat on an internal webserver. That also runs java servlets and other applications. It connects directly to our SDE.
That application is accessed internally, and through a Web Application Firewall (tomcat running modproxy) for external; public and secure access.
The advantage of this architecture is we have a single app, single development environment, single webserver, etc..
(Downside is single point of failure, but redundancy/HA could be easily achieved if we had hardware funds).
Choosing the Javascript API allowed us to not have to worry about platforms/plugins, etc.. And it integrates into our existing JAVA/Tomcat architecture seamlessly.
Plus, if we ever do go to IE9, all i have to do is change the DOCTYPE. I have in fact already done this using quirks mode in IE7 and it seems to be working great!
Long story short is the Javascript API requires the least setup for the end user. Open browser app works. No plugin download, no patches/security concerns, and free development tools for me. Win Win