I'd not want an update all per se, but how about an "update group" functionality? So you could create a category like you do now, but have an option to "choose all" from the category. Or, tick boxes for each entry in a category to install at one time. Agree that it could be better in this regard.
I get the nature of this request, but its important to keep in mind that the Self Service.app is more than just for installing or updating software. Since its really a policy runner, the available or scoped policies could range from updating a piece of software to flushing system or font caches, deleting printers, installing Configuration Profiles, make me an admin for 30 minutes, and much more. Just take a look at any of the show off your Self Service threads here and you'll see a very diverse use for the application, not just for installing a software title.
Contrast this with the iOS app Store, which is really just for installing apps, and the Mac App Store, which is for installing/updating apps and System software. So in actuality, Self Service is more versatile than either of the other products (in that regard at least)
I agree with @boettchs that I wouldn't want a button that just ran all policies in scope for the above reason. But I think he also has the right idea. Either a built-in category that you can place policies into, or a checkbox per policy, to make them appear in a category that then has a "Run All" button on it might be nice.
Oddly enough, this was more of less the idea and motive behind my Feature Request here: https://jamfnation.jamfsoftware.com/featureRequest.html?id=1968 Basically, I wanted a simple way to know which policies were in scope for a Mac via the binary or the API so I could build a multi policy run function. Something that would let you run several policies in succession.