I've subscribed for one month only on several occasions. Usually when something new is added that is relevant to one of my lines, eg a new set of parish registers. Or if I've discovered a new lead. As lilly14 says, you have access to all the features of the annual sub.
As a bonus, when I've cancelled after a month, I've sometimes been offered a very good deal to continue, such as four months for £20.
The library is great if you just want to check a few things, but there's no comparison to having access at all hours at home.