such an app would make the entire ruleset available with a few menu button taps, yes. but how much would be immediately relevant? for example, what would you include that would not fit onto creativehum's dm screen? what exactly is to be gained by such an app?
It gives the user the rules in electronic format. Lots of stuff is being sold as a PDF today, and, frankly, I doubt most of it is being printed out. PDF is "OK", and suffers from it's limitations. Try the enormous Starfire Solar Rule PDF. It's, what, 300-400 pages, lots of internal links, and they did a great job, but it still suffers, especially on a touch device. I'd love to be able to mouse over an abbreviation and get its definition right there.
But not just electronic format, but ideally a first class rules format, ideally something better than just links, contents, index, and search. For example, on a iPhone with "3D Touch", you can see a rule reference in the text: "Bullets do stated damage, unless the attack is Dodged, see
(Rule 1.23)" and press over the link, and a summary of the rule shows up. If they tap, they jump to the rule, but the summary may well be enough.
You can have an active link: "Players who drink poison must make a
Saving Throw against Constitution or suffer
2D6 of damage." Tap the Saving Throw button, and the dice roller app pops up, with the last Character (or NPC) in context (which may or may not be relevant, but the other PCs are right there), so you can tap the "Saving Throw" button for that character. 3D Touch the 2D6, and you get an instant roll.
When you update the "app", you get the Errata incorporated instantly, and correctly (i.e. the indexes are updated, etc.). Of course, you can annotate the rules as well with your own notes. Add your own footnotes, etc.
Ever read a GURPs book that references the Basic Set (or even another GURPs Book), those are all integrated seamlessly (assuming you purchased the relevant modules).
Of course, you have the Char Gen app, the Ship Design App, the Traders Worksheet, a "Play Session" page keeping track of characters, and combat, offering dice rolls, character sheets. Have the player tap the "Roll to hit" button on their app, and the ref gets not just the roll but whether it succeeded (and how well it succeeded). Up to the ref to decide if it DID succeed, but he can see the results of the roll, not just the actual number. He can even set up the context, so that there might be hidden DMs the player doesn't know about.
The ref can stand up combat by selecting the characters, and the relevant NPCs (since they're annotated in the game module): "
Starport Bar: It's bustling, late afternoon, happy hour is just starting. If the players remain more than 1/2 hour, Bounty Hunters
Rocko and
Tex will show up looking for them." 3D Rocko or Tex, and get a quick summary. Click on the Starport Bar, and a map, with the NPCs and the players, may show up, ready for action.
So you end up with an integrated, extensible rules system. Adventures can integrate in to the environment. "Tex will grapple
(Rule 2.34) the closest player." Never Grappled before? Well there's a handy reference right in the adventure.
There are all sort of opportunities available with the new medium. It's certainly not easy, but it's available. Over time, perhaps the ref won't need much more than a GM screen, but that doesn't mean that Play Management utilities won't be helpful in the long term.