Re-using soil-less mix for seed starting would probably promote damping off, which occurs when fungi in the soil-less mix attack the tender baby stem right AT the soil line. Usually or always fatal.
If you NEVER have damping-off in seedling trays, but want more of a challenge, you COULD try that. But I wouldn't.
As to re-using it for potted plants, I just don't know. But beware of re-using some mix that was "fragile" and broke down or decomposed part way.
That would reduce the drainage and aeration of the mix by making it finer. If air can't get to the roots, you'll wish you had used a coarser mix when the roots rot.
