This is a bad thing. Gmail needs to be e-mail and nothing more or this is EEE.
I totally agree. Besides, what is wrong with HTML? And why doesn't it render properly in the gmail client?
This way they can use a more data-friendly format created and supported by multiple organizations (Schema.org), which any client can use freely.
As a non-Gmail user myself, I'm quite happy with their choice, since it means I can make it work with my client and my calendaring software.