Dagney had no practical use for a smelter, but Hank Reardon did.
It's also unlikely that she would've stolen what was clearly Francisco's intellectual property in order to give it to someone else. That clearly would've been morally different than, for example, paying Quentin Daniels to reverse-engineer the abandoned motor (especially given that she believed that the inventor was likely dead). Even though she was a scab, the strikers still believed that she was moral. John Galt even told her the password to the power plant (which was the oath that everyone took to join the strike) because he knew that she wouldn't actually say that until she said it in the sense that he meant it.
On the other hand, Dagney wasn't permitted to attend John Galt's physics lectures (or other meetings that centered on exchanging useful ideas and information) because that was not intellectual property. There was no reason she couldn't use that information to benefit Taggart Transcontinental and/or other people (such as Hank Reardon) who were still "on the outside".
Disclaimer: see this question for details on whether Dagney had a practical use for the content of John Galt's physics lectures.
