I’ve been involved in a few discussion with people at Microsoft and it seems there has been a swing in sentiment with regards to whether customers should use Microsoft Peering over Express route at all. The long and short of Microsoft Peering is that it enables several Azure services to be available over Express route rather than the general internet. This includes Exchange Online, SharePoint Online, Skype for Business, and Dynamics 36. It excludes services such as Azure websites and Azure storage as these are available over ‘Public Peering’ instead.
When we first deployed Office365 (probably a good 4 years ago now I think?) we saw a perceivable increase in performance when running over Express Route rather than the internet – Our PA’s were running Outlook in the non-cached mode due to the number of mailboxes and calendars they were looking after. Express route made a big difference here.
It’s hard to say at this point why the recommendation would have changed from a reliable path (Express route) to big bad internet routing. To be fair, Microsoft would probably peer with most major ISPs in some way or another so maybe that’s where they are coming from (that it’ll effectively be the same number of hops there anyway?). It’s still a contended, and non QOS supporting path.
I see some murmurings on the internet that they are looking to combine the Public and Microsoft peering into a single routing domain. I’d welcome this as it means a few less BGP peering points. All of the prefixes they advertise now have BGP communities so it’s easy enough to distinguish different services being advertise over the same peering.