This would be a great feature to add. When writing an email you have to click the spell check button. This should be automatic like other email providers.
It is a good idea for Method:New and is available for Method:Classic. I would suspect this came up when they were looking for HTML Editors to use with Method:New. The issue is the HTML Editor is done by a third-party software developer. Most companies do use another company's HTML Editor because of the complexities of building your own HTML editor. (This is also the case for the Report Designer). Our hands are tied unless the HTML editor developer updates the software or Method goes with another HTML Editor.