I think you may be a little confused on the Office 365 "Subscription". You don't need the subscription for the phone but this depends on what your purpose is. (Business or Home Premium). MS has really murked up the Office 365. There are several different flavors of the Business side. I have a P1 plan. Website, sharepoint online, exchange online. I also get the Office web apps. Its almost like signing into Outlook.com and having access to Word, Excel Online, OneDrive etc. They also have the Office 365 Home Premium. This gives you 5 licenses to Word, Excel, PowerPoint, OneNote, Outlook Publisher full applications for 99 bucks. Its for the home user that has a desktop, laptop, ipad, and perhaps one of the thousand droid tablets (Or a Surface Pro). With the Home subscription you can install the full office suite onto you desktop, the mac, an ipad... use up three of the 5 license and have 2 left. You DO NOT need to use on on the phone because these are already in the phone. The home subscription also gives you more OneDrive storage space and Skype credits.
Bottom line, if you have the home premium office 365 subscription... this does not have any effect on a windows phone, or Surface RT (Office is already on these devices). If you have an Office 365 Business plan, now it gets interesting and the phone will have the capability to access SharePoint online. You can get access to you business document libraries. WP Word, Excel etc work just the same, just that you now have access to a new location on the phone called Office 365. When you see office 365 on Windows Phone.... IT IS BUSINESS ONLY. The Home Premium subscription of Office 365 is based on you MICROSOFT ACCOUNT and a Windows Phone is already tied to your MS account.