Let me start by saying that I don’t have yichus, am not married to someone with yichus, and don’t really think that yichus is very important.
From my understanding of the issue, however, people who think yichus is important do so because of the following reasons:
– They want to make sure that there is no p’gam in the ancestry which may have happened if there are ancestors who were not shomer Torah U’mitzvos
– In cases of close yichus, as in father, grandfather, and perhaps great-grandfather, one would hope that being raised in a family that is known for excelling in limud Torah, yiras shamayim, middos tovos, etc would have had an effect on the child in question and would produce a highly mechunach and refined ben torah. Of course, as a previous poster mentioned, if that proves not to be the case, this reason would not hold water.
– Zchus avos. There is a concept of the descendants of a great tzaddik having special shemirah and zechusim based on the merits of their ancestors, assuming that they themselves are a decent person.
I would be surprised if someone took a shidduch based on yichus “at the expense” of other crucial maalos – most people sould looking for yichus in addition to other wonderful things. And of course everyone has to prioritize all the things they’re looking for. Just like everything else on the list.