There really are a lot of good reasons to ally the new Space Marines with your Imperial Guard. For myself and my Imperial Guard however, I am saying no thanks. There are three very good reasons why, and in the video and below, I will go over those.
Dark Angels are my choice of allies for my Imperial Guard, and here is the brief on it....
1. Librarians and Powerfield Generators. Yes, thats right, 4+ invul saves to any model within 3 inches. I care not that I cannot be in a vehicle to provide this bonus, as it still means that focus must be spent to target my Librarian or Librarians (in 2000+ pts).
A single powerfield generator can very easily cover 3-4 tanks, and with how many ways there are now to get rid of cover, very little gets rid of a 4+ invul save.
Librarians get Diviniation. Thats right, codex space marine Librarians do not get the Divination discipline, but Dark Angels do. So yes, those basilisks or Collossus barrages can re-roll to hit, not to mention anything else, as the power has a 12" range.
2. The Inner Circle: Thats right, not only is the librarian inexpensive when it comes to points, he has the Inner Circle Dark Angels special ability. This gives the librarian fearless and preferred enemy (chaos space marines). A 20+ man infantry squad of guardsmen with a single librarian attached is now fearless. wow. They are going nowhere, not to mention the librarian comes with all the abilites and gear listed in reason 1. This saves you points, no longer needing commissars in that big blob, as the librarian and a vox caster for orders will do just fine.
3. Techmarines. Just in case you did not know, you can take a Dark Angel Techmarine for every Dark Angel HQ choice. That means up to two as allies in larger games, one in small games. This Techmarine can take special issue wargear....... which amazingly enough includes a powerfield generator. Did I mention that he can bolster defenses and fix the tanks he protects with a 4+ invul? So he generates a 4+ invul save, and can fix damage on the tanks he protects.
There ya go. Imperial Guard players may already know all of this, but with all the new hype going on right now about Space Marines, Eldar and Tau, I thought I would show a divergent way of thinking for your lists.