Well normally people, such as myself, use sales strongly in a debate because they are the final say of whether a not a game or hardware can be deemed a success. However, considering we don't know what the companies would consider a success for the most part they can be pretty worthless. Thus the only way to tell is through assumptions (usually by most are pretty poor ones) and historical trends. For example, if people actually compared now to previous times in the Japanese markets, I don't think people would be freaking out as much.
So why do people use them so seriously? Because it does matter and it is something finite and concrete you can use. Brand nationalism, although more interesting to most and more fanatic (to your disbelief), is mainly normative. There is nothing you can really prove or settle with it. Instead it leads to ongoing conflict with absolute no end. At least with sales data you can put an end to some things... although not all. Thus it plays quite an important part in settling disputes. Not to mention this is a sales site haha meaning it will take up a lot of discussion.
Sales do matter but people must understand that to use they don't mean everything. I mean I'm disappointed by the sales of a lot of games but I'm more ecstatic that I did get to play it. People can let sales devoid them from their reality, which is actually playing and enjoying the games that the businesses provide. Sales analysis from a consumer standpoint is somewhat ironic but to some it is quite a fun thing to do and for the most part can lead to enjoyable debate. Just take out the biases and hatred and it will do just that... can't have that en the battles of opinions.
So that's why it matters but also why it shouldn't be the only thing it matters.
