> He seems to reject the premise on the grounds that it's sometimes inconvenient
His argument is that if you consider dual licensing to be immoral because it enables the development of proprietary software, then to be logically consistent, you must also consider non-copyleft FOSS licenses such as BSD/MIT to be immoral because they do the same to an even greater extent. Stallman does not say that dual licensing undermines free software, he only says that it does not support free software as much as copyleft licensing does.
The FSF only uses copyleft licenses for software instead of non-copyleft FOSS licenses or dual licensing, sidestepping this issue entirely. They don't have a problem if people use BSD/MIT, so they also don't have a problem if people use dual licensing.
His argument is that if you consider dual licensing to be immoral because it enables the development of proprietary software, then to be logically consistent, you must also consider non-copyleft FOSS licenses such as BSD/MIT to be immoral because they do the same to an even greater extent. Stallman does not say that dual licensing undermines free software, he only says that it does not support free software as much as copyleft licensing does.
The FSF only uses copyleft licenses for software instead of non-copyleft FOSS licenses or dual licensing, sidestepping this issue entirely. They don't have a problem if people use BSD/MIT, so they also don't have a problem if people use dual licensing.