Ultimately, if you are looking curiously at Office for iPad and already have an Office 365 subscription then go ahead and download the apps – they are part of the package and it doesn't make sense not to use them, even if it is very infrequently.
However, if you are going to have to sign up for Office 365 in order to use Office for iPad then it would at least be worth trying Apple's iWork suite beforehand. If there are some Office-specific features you need missing – which may be the case with PowerPoint or Excel – then you'll know that spending £7.99 per month on Office 365 isn't wasted money, and it enables the download of clients on Mac and PC, as well as syncing between them all.