Any tablet can already create documents and spreadsheets, surf the internet, check e-mail and calendars, access social networks, edit movies, Skype... Everything that %95 of the population needs. People that need higher caliber tools (developers, creative professionals) can afford another system.
So Windows needs a stronger reason to exist it's going to dissuade the average consumer from buying either the market leader (Apple) or the cheap alternative (Android). Android is the real threat. A $200 PC replacement is quite feasible within a year and really deals a blow to Microsoft's business model.
For consumers its less clear. Maybe XBox Live.
Any tablet can create spreadsheets, but not as well as Excel. Any tablet can create a document, but not one that's going to be 100% compatible with the version of Word they have on their desktop PC. Those are all very strong reasons to buy a Windows Tablet.
Sarcasm aside, a lot of people still use Office. Far more than use Google Docs. Even businesses that are using Google Apps for e-mail still use Office for their documents. Being able to edit documents with 100% reliability on a tablet could be a big deal when it comes to corporate tablet purchases, for a start.
"Microsoft’s average revenue from its Office suite licenses sold to enterprise and retail customers has been declining due to competition from cloud-based app providers like Google, Amazon and Salesforce.com, and also due to increasing lower priced products sales in emerging markets."
There are still a ton of companies out there that use office. Creating a Windows Tablet could well cause sales of Office to increase.
If for some reason, you need another edition of the development, you can get them free if you're a student (DreamSpark), a startup (BizSpark), or cheaper than $700 (MSDN sub).
I'd be interested to know why Express doesn't work for you?
You can even hook up a second monitor to it.