If I take the answer for that forum post as valid
Then Microsoft.Web.Helpers are simply experimental helpers that microsoft didn't include in the "official" ASP.NET MVC release and thus won't support. However they still put them out there so that it can benefit those that don't need full support.
There is nothing wrong in using them, its just that you won't be able to call Microsoft tech support if they don't work. You can probably fill bug report on them and have the community answer your questions anyway. What you won't have is "Call us at midnight on Christmas Eve" support. Some organization have a real issues with unsupported components, thus the separation between the official release and this.
You can seem them as kind of compromise between MS big corporate customer that require support at all cost and the need of a small start-up that would switch to more nimble cool new framework otherwise. Also, keeping the two separate prevent bloating in the official release. Essential stuff in System.Web, additionnal goodies in Microsoft.Web. Not everyone need Gravatar support on his website.