Yes, you just produce less entropy. But you produce a lot less entropy and it is completely unrelated to Landauer's principle.
Suppose I want to calculate a 1TB document created by a googol person civilization running for a googol googol years. I only have to produce a TB of entropy, not more than a googol googol googol bits (as I would have to if I used irreversible computing naively).
Very nice! So you don't just cheaply compute-and-uncompute a lot of independent worlds, you can allow them to leave an arbitrarily-difficult-to-produce trace on the future worlds. Given how much entropy we really have, sufficiently small persons for example can be spared from uncomputation.
In particular, a person can live in an incrementally computed-and-uncomputed virtual world that is being regularly reversed to its initial state, with the effect that only the person consumes entropy, and the whole arbitrarily complicated world has zero entropic footprin...
This post may be interesting to some LWers.
In summary: it looks like our universe can support reversible computers which don't create entropy. Reversible computers can simulate irreversible computers, with pretty mild time and space blowup. So if moral value comes from computation, negentropy probably won't be such an important resource for distant future folks, and if the universe lasts a long time we may be able to simulate astronomically long-lived civilizations (easily 10^(10^25) clock cycles, using current estimates and neglecting other obstructions).
Has this been discussed before, and/or is there some reason that it doesn't work or isn't relevant? I suspect that this consideration won't matter in the long run, but it is at least interesting and seems to significantly deflate (long-run) concerns about entropy.