As I have thought about my latest Sanguinary Guard list, I have decided that though Mephiston is great, I don't think that he is part of a more competitive SG army. If you have to have Dante to make SG troops, then you are already looking at 225pt. in HQ slots. That means that while Mephi is fun, I don't think he will make a competitive edition to the army.
So, this leaves me with two options: Drop Mephi for more models, or drop him for a libby and more models. I think the latter is the best choice.
You see, Jump-ish (my term for mostly DoA armies) armies and DoA armies really benefit from the libby. He brings so many good things to the table. Kirby wrote that he is one of the best force multipliers the army has aside from sanguinary priests. In my mind, libbies are almost an auto-include for BA jumper armies. Here's why in no particular order.
1) Pyschic protection. The hood is vital. BA, SG armies especially, are model-lite. This ultimately means that you need to protect the models you have. With the recent release of GK and the presence of Space Wolves and such, a hood provides a much needed first line of defense for your army against those psychic powers. This is huge. Also, having Dante in your army can build synergy in this regard. Use his death mask to lower the stats of your opponent's libby and you will stand a better chance of shutting down his powers if he perils or is simply wounded as most libbies have only 2 wounds.
3) Diversity. Depending on your psychic power loadout, your libby can help handle a multitude of situations. My two favorites for my SG army are Sanguine Sword and Blood Lance. Both provide offensive power that will help clear out tanks and infantry alike. I don't include Unleash Rage here because of the prevalence of master crafted weaponry in my SG army. However, if you are running a DoA army that focuses more on assault squads, this power is golden. I could care less for the rest outside of these four powers.
4) Cheap. This is as cheap a character as you will get to lead your forces. If you are running a DoA army focusing on assault squads, he is the way to go for the output and utility he provides.
So, don't leave home without your librarian. He is the way to go.
P.S. I looked for a decently painted jump libby to put in the post and couldn't find one! Maybe it's me being lazy, but if you have a decent pic of a well painted libby, link it in a comment for me. Thanks!