@Xenoscream
I've been a video game tester for ~6-7 years now. I'm done with game testing at this point, but I did learn a lot.
A day one patch is usually due to new issues being found later in the development cycle. Throughout a project it's quite rare that you don't find one or two serious issues in the months leading to release. At this point in the project, builds have already been submitted (or are about to be submitted) to Sony and Microsoft for certification. If Cert finds an issue, the option to resolve those issues in a day 1 patch as opposed to delaying the game is there.
It is a bit of a nuisance for gamers, but not a huge one.
Though I do hope that Alien: Isolation isn't too buggy on release.