First of all, the question of authorship doesn't necessarily affect the Zohar's importance in halakha; I'm not sure why you think that this it would "obviously affect the way of poskining". The question of how Kabbalah affects Halacha is not a simple one, and does not really depend on the Zohar's authorship, because even if it were written by a Tanna, we don't just hold like any Tanna in halakha (after all, how many Toseftas are there that we don't hold of!). This is discussed on this site, by this audio shiur and in the book העד הגל הזה, where R. Yaakov Hillel has a great chapter on the topic.
However, there is more to Judaism than halakha. The way that we understand certain views or 'hashkafos' to be 'Jewish views' or 'Jewish values' has a lot to do with the understanding of earlier Jewish sources. 'Jewish sources', though, are both vast in number and can vary widely vary in their positions, and equal weight isn't given to every Rabbi who happen to have written a book. There are a couple of factors, therefore, that would cause one source to be more 'weighty' than another, authorship and age being two of the major ones.
If a particular person is a great Torah scholar or known to have a much deeper knowledge of Judaism, than his opinion of what the Torah's position is on a particular issue should be given more weight than that of a lesser scholar. Thus, the Tanna R. Shimon bar Yochai is well known to us as well as to the members of his own time period as a great scholar, a very high-ranking Talmid Chacham, so to speak, as well as an incredibly righteous person, so much so that the authors of the Gemara felt that it was fit to record (in Sukkah 45b) that R. Shimon bar Yohai felt that if there are only two great people at all, it would be himself and his son. The other contender for authorship, R. Moses de Leon, is not nearly as highly regarded, and therefore if the Zohar reflected his opinions, they wouldn't carry as much weight.
Additionally, for whatever the reason, when it comes to evaluating the importance of a given viewpoint among the sea of Jewish authors, earlier sources are given more weight than later ones. This rule is fairly well-accepted to in halakha, though of course there are exceptions, and so many[who?] apply the rule to hashkafa or theology as well. Thus, even disregarding the discrepancies in the scholarship level, then all else being equal, the relative importance of earlier sources is higher than that of later sources.
Finally, there is the secondary issue of what it would mean for kabbalah that R. Shimon bar Yochai did not write the Zohar. The word kabbalah means 'a recieved [tradition]', and the general authority (or authenticity, or general value) to kabbalah, as opposed to any other theosophic system that would be attributed to it by religious Jews is because it represents a received tradition that traces itself back to something revealed by God, which is why we value the Torah and deem it authoritative. Hence, while it's true that some have denied the Zohar's tannaitic authorship while maintaining it's essential nature as a text of authentic Jewish/divine presentation, (such as R. Eliyhu ben Amozeg, and to an extent, R. Yaakov Emden) overall, to say that a work of kabbalah which is so vast and attributes so many ideas to Tannaim is actually of a much later composition is anathema to the whole spirit of kabbalah, which is supposed to be grounded in 'received traditions'.
Incidentally, R. Chaim Kanievsky is quoted as saying that someone who doesn't believe that the Zohar was actually authored by R. Shimon bar Yochai - seemingly, even if he believes in its authority as a Jewish text - is a heretic (Vayichtov Mordechai pg. 340). For further reading, I'd suggest this article which details many Rabbinic opinions regarding the Zohar's authorship, without actually discussing any arguments for or against, while touching on these questions indirectly. Note especially footnote 8, where the point is made that disputes about the Zohar's authorship are really made against the backdrop of it's acceptance as an authoritative Jewish position.