The officers were paroled, and without any ransom as late as at start of WW1. For example the later marshal Tuchachevsky was a "poruchik" (senior lieutenant) then and was taken as a prisoner by Germans. As with all other officers, he was allowed to walk into the town and had his freedom, only he gave his honest word that he'll return into the barracks.
But he tried to escape. 5 times! The fifth attempt was successful. You can imagine the "strictness" of the guard! Only after the first attempt to escape and a mutiny he was forbidden to have walks in the town!
So, the parole system worked. But for officers only.
As for the system "free for a promise not to fight", the last war that used it, as far as I know, was the Russian-Japan war of 1904/5. And it worked these times.
The same system was tried tofor use by Chechens in the first year of the first Russian-Chechen war (1995-98). But then it didn't work. The fact of "word" was irrelevant to the contemporary Russian state machine.
So, the question is not in acceptability, but in usability of the practice.