Not seeing the point here. There are a lot of things that have expiration dates, including IRS refund checks from the government. Why bother registering if you're not going to take the test in the immediate future?
To address the last sentence first, some people register well in advance of the planned exam date, using the exam as a commitment device. Six months is not that long, but you can plan if you know what the deadline is. Let's take a look at that refund check: if the expiry date prominent, or is it on the reverse, in font size 4?
(Things are better now, with the expiry date added to "Exam dashboard", but note that even now, you only learn about the deadline *after* you register - and the fee is non-refundable, by the way. Loyalist are not doing a good job there, and Tableau cert should have stepped in - but hey, they did not even bother to fix the errors in their online sample exam, which I reported months ago).
... By way of an update, I tried to pursue my complaint by writing (a) to firstname.lastname@example.org, (b) Tableau's EMEA office, located a few miles from me. I have had no response to either letter. This reinforces a negative impression of the company.