This is probably down to personal preference, but I find floating fixed-position toolbars rather intrusive and reduces the usable real-estate in my browser.
I remove as many toolbars as I can from the browser (bookmarks, addons, etc...) and it would irk me if a website then forces one on me.
In the rare occasion that I need to access the top links while I'm deep in a page, I'd much rather have a shortcut key that scrolls me to the top of the page. For now, I'm doing Ctrl+L then ENTER which reloads the page (thus grabbing new notifications) and put me back where I started.
As pointed out by Bobby in the comments, Ctrl+Home scolls to the top of the page which is also useful since it doesn't require reloading the page and will still work if the URL navigates you to mid-page.
p.s. the initial GM script that predates the StackExchange™ MultiCollider SuperDropdown™ had position:fixed
, but there was some objection to that positioning. IMHO, you'd be hard pressed to convince the dev/design team to make the toolbar be displayed as such.