I don't know of any real cons,,,,
The Pro's are numerous....
Although this really only works in IE.....
Access to OneDrive without having to log in
Access to your Address book, Outlook Online and numerous other Office On-line apps without having to log in.
System stuff....
Sync Settings and Passwords, themes (colors, backgrounds, etc,) and other things (IE Favorits and Tabs) across devices.
App Settings, such as a list of apps you have installed from the store.
So, say you install Solitaire on your Laptop when you are somewhere else
You come home and want it on your Desktop (or other device)
Log in, and the icon will be there,, click it and it will install without you having to search for it.
Some App settings will transfer across devices as well.
If you use Office 2013, it will stay logged in and you can sync docs, and OneNote across devices as well.
There might even be other stuff I am not mentioning.
But soon, as Universal Apps gain momentum and
now that MS has acquired GreenButton and they will incorporate that tech into Azure
We will see even more apps and data for MS Devices that will sync Across Devices (WP, xBox, Desktop, Tablet, Laptop).
These next couple of years are going to be pretty exciting.
I am actually on board with what MS is doing right now. I was against cloud services,, but for now,, things are fine.
I would recommend encrypting financial and personal data if you are going to store on OneDrive, DropBox, etc. as in any cloud storage service.
Not applying it to only MS, but over all in general.
Keep in mind, that if you work in Word, Excel etc. You do not Have to save it in the cloud
You can save it on your PC and never touch the cloud.
There are still some advantages though to being logged in as your Main account.
OneNote for notes of any kind (such as I use it for recipes, Windows Information, Technical Info) to have it on all my devices.
Edit/Add once and sync, done.