When I originally developed dismissible admin notices with Helen my first patch added the ability to hide the notice for a period of time with JS cookies. Unfortunately, we were in beta and could only fix bugs and this functionality was an enhancement. I probably should have followed up with my original patch and then it could have been in Core already. I would likely still use only JS though. Making an Ajax call to PHP to store a transient about a notice to me seems overkill. Regardless of the difference of opinion if anyone wants this sort of functionally in Core they can always open a new Trac ticket with a patch, or comment on the original and include the patch there. Also, the earliest it would be in Core is version 4.7 because 4.6 is currently in beta.

https://core.trac.wordpress.org/ticket/31233