In the traditional colony-ship scenario you'll have had to solve the self-sufficiency problem. So additional matter and energy will only be used and run out inasmuch as you're developing and creating new things beyond the originally-conceived equilibrium point of your ship.
As that's likely something an inventive race will always do, I'll grant matter and energy exhaustion even though it wouldn't be in the form of 'running low on food/fuel'.
But even then, matter and energy are far more safely and easily found, harvested and retrieved if you never enter any planet's gravity well.
If you're thinking more along the lines of a hyper-space scenario, where FTL technology is achieved before self-sufficiency is solved, sure, you'd have more-traditional 'supplies' and need to land and colonize Earth-like planets.
We've been referring to different things. Since you referred to "indefinite space travel" I thought you were talking about not going to another solar system at all, i.e. that "land" meant orbiting another sun.
But that was obviously a misreading on my part. Anyway, I completely agree with you that once your ark ship arrives in a new solar system landing on a planet like Gliese 581g would be counterproductive.
Instead of having to deal with a gravity well it would be better to make a home in something like the asteroid belt. You only need energy & resources for manufacturing capacity, a gravity well can only complicate that process.
People might still want to live on planets as a novelty, or for their resources. But I think any society sufficiently advanced to develop a generational ship that can traverse interstellar space would resemble The Culture (i.e. a swarm of migrating ships & habitats) more than they would resemble The Federation (i.e. people mostly living on planets with ships mainly for transport).
As that's likely something an inventive race will always do, I'll grant matter and energy exhaustion even though it wouldn't be in the form of 'running low on food/fuel'.
But even then, matter and energy are far more safely and easily found, harvested and retrieved if you never enter any planet's gravity well.
If you're thinking more along the lines of a hyper-space scenario, where FTL technology is achieved before self-sufficiency is solved, sure, you'd have more-traditional 'supplies' and need to land and colonize Earth-like planets.