Though the game does provide alternatives - a hearthstone equivalent and an NPC teleport service - the main reason why I'm still sitting on all of my runes is that it is very rare that I ever feel I *need* to backtrack. I travel to a quest area, I complete all the local quests (or maybe all but one or two group quests that I will come back to solo at some higher level), and I leave.
Why Backtrack?
This particular feature of games is by no means unique to ROM - the unused teleport runes just happen to be an especially noticeable way of keeping "score" to show just how little use I have had for worldwide travel on a scale that most games do not offer.
Pretty much every game offers some reason to go back to quests that you have yet to complete (assuming that there actually are enough quests that you have extras left over, which looks like it will no longer be true of ROM by the 30's). In ROM the reason is experience for your second class, while WoW has its achievements, EQ2 has AA's, LOTRO has deeds, and DDO has favor. Once you've completed the quests once, though, it's usually only a small subset of repeatable endgame content that actually provides any real push to return.
(DDO is an exception because literally all of its content can be repeated on higher difficulties for additional favor - in my view, this aspect of the game is part of what makes the purchase of adventure packs feel more compelling than even larger amounts of content sold in paid expansions for other games.)
The world behind
This aspect of vertical advancement - complete one zone and move on - is creating some of the genre's biggest challenges these days. It is very challenging for developers to provide any kind of group leveling path because the inexorable upward movement of the player population means that there won't be anyone left at the lower levels to group with. It is challenging for atmosphere, as supposedly remote areas are overrun on expansion launch day and populous towns are deserted a year later. This approach is also clearly taxing development budgets, as it calls for more and more content at a faster rate than even the largest studios can sustain.
At the end of the day, perhaps the main solution will have to be providing compelling differences in gameplay (either revised quests or compelling class choices for alts) in the hopes that players will re-roll. It will be very interesting to see what Cataclysm - with possibly the largest scale revamp of a leveling game ever to hit an MMO - does to WoW's demographics. Will longtime players re-roll, zoom through at 85 for achievements, or just wonder why this feels like a shorter expansion than some? That said, it wouldn't be all bad to have some reason to revisit locations in the world beyond the occasional Fed-ex quest.