From a control/compliance POV, a pitiful client like a browser is probably just the job though. And it does render pivot tables and charts well.
The problem is getting developer buy in, same as VSTO. They are great technologies that can add real business value, but .net devs aren’t interested in Excel, and Excel devs aren’t normally empowered to use these tools.
Excel Services could change that though – as Excel devs can publish their workbooks directly to the server straight away, and as time goes on make a case for getting Visual Studio and VSTO.
Maybe this could be the bridging technology?