A ton of things are on a spectrum where you don't want too little or too much. People can die from having too little water or too much water, too much heat or too little heat, etc. A lot of people like games that are within a certain difficulty range rather than being too easy or too hard. Different people can have different preferences. I don't know what specific slider weight would be optimal, but I imagine it's potentially somewhere in the ballpark of the weights that we give the other factors.
@FtXCommando
The slider would still have an impact. Being able to set one's preferance for that would be a nice QoL improvement that would allow people to choose their preference on the spectrum of better game quality vs faster matching. We already have people being confused and frustrated about not getting a match. This wouldn't create that problem. That problem exists either way. A little info blurb stating that using the slider makes your preference more likely but not guaranteed, could be added if people really want...
I'm not claiming that example A or example B are perfect, just that I think the system could be better with either of them than it is without them as it is now. There are pros and cons for each of them compared with each other and compared with the current system. I am inclined to think the pros outweigh the cons for either of them.