To me, Dungeoneering is the second major indication that the folks at Jagex could be at least preparing the way to make a transition to a F2P cash shop for Runescape.
The first big signal was that "Bonus XP" weekend, wherein players were granted something like 10 hours of in-game time of higher-than-usual XP gain. Bonus XP is a standard cash shop offering in most F2P games. One can easily envision, now that they've tested the concept, that this could be in a future RS cash shop.
Now Dungeoneering comes along, and it screams out to me: Cash Shop Framework! We now have the cash shop currency in place: Dungeoneering Tokens. Just looking at it I'm envisioning Dungeoneering token prices being something akin to $1/10,000 tokens.
The Dungeoneering rewards mimic the kinds of things you'd find in a F2P cash shop:
- additional inventory slots (gem bags, etc.)
- degradable top-tier-items that require "recharging" (i.e., recurring purchases);
- one-time purchases that give you an in-game advantage (scrolls)
The kicker: Dungeoneering XP can be purchased at 1XP per token. If you want to speed your way to using these new items ASAP, then get ready to pay $1 per 10K XP. You can't buy the other skills, mind you, but you can buy Dungeoneering XP which goes up to 104 million XP to reach level 120. Leaves plenty of purchasing room, and explains why this one skill goes up to 120 instead of 99.
So in conclusion, I suspect that the "skill" Dungeoneering is far worse than being nothing more than a minigame. It's the infrastructure for moving Runescape in the direction of a F2P cash shop. Our best hope is that they're just testing the infrastructure here in RS, but will only be implementing the cash shop in Mechscape.
EDIT: And the more I think about it, I could definitely see some future in-game content inaccessible to those without a Dugeoneering level of, say, 110, 115, etc. A level that would take an inexplicable amount of in-game time to achieve, so it would essentially amount to content that you'd be required to pay for (since you'd have to buy the XP).