What is the exact sentiment expressed by も in this sentence as opposed to が?
It's a sentence in response to someone saying "I saw a spider in my room, I was scared".
も is used instead of が to add the meaning of "even" or "also." See for comparison the following examples:
In Japan, there are people who say that seeing a spider is a good thing.
In Japan, there are also people who say that seeing a spider is a good thing.
Depending on context, one can also translate も as "even," as Istraci does:
In Japan, there are even people who say that seeing a spider is a good thing.
So も introduces an element of comparison, implying that there are people who think that seeing a spider may be a good thing, as well as those who think that it is a bad thing. In natural colloquial English, perhaps one could drop the "even" or "also," as these seem somewhat stilted in the context of a conversation, and introduce a word like "well" instead:
I got scared because I saw a spider in my room!
Well, in Japan there are people who say that seeing a spider is a good thing.
It's saying "There are even people who say..." Since it's not really the norm to think that spiders are a good thing, it's emphasizing that there are some who do think so. が would work fine as well, but the も gives it the emphasis that even though this thing is unexpected or in the minority, there are some people who take that side.