שָׁאֲלוּ אֶת רַבִּי אֱלִיעֶזֶר, מִי שֶׁעָלְתָה לְתוֹךְ יָדוֹ בַּהֶרֶת כַּסֶּלַע וּמְקוֹמָהּ צָרֶבֶת שְׁחִין. אָמַר לָהֶם, יַסְגִּיר. אָמְרוּ לוֹ, לָמָּה, לְגַדֵּל שֵׂעָר לָבָן אֵינָהּ רְאוּיָה, וּלְפִשְׂיוֹן אֵינָהּ פּוֹשָׂה, וּלְמִחְיָה אֵינָהּ מִטַּמָּא.
They asked R. Eliezer: "[What is the ruling concerning] one who had a bright spot the size of a sela form on the inside of his hand and it covered up the scar of a boil?" He replied: "He should be isolated." They said to him: "Why? Since it is neither capable of growing white hair nor can it effectively spread nor does quick flesh cause it to be unclean?"
Leaving aside R. Eliezer's subsequent answer and the ensuing discussion, I want to probe the premise of the original question.
It seems to assume that if one has a nega' with no possibility of declaring it to be definitively impure (hechlet) then there is no need to quarantine it (hesger) in the first place.
While I can understand the logic of this approach (in that the point of quarantining a nega' is to see whether it will develop in a way that it can be ruled definitively impure) how were the Rabbis involved in this discussion so sure of this premise?
I could also have understood that the Torah requires that a nega' which appears in a certain manner be quarantined, even if we know for sure that it could never develop into a definitively impure nega'. Why were the Rabbis sure that this is not an option?