I believe familiars are constitutionally unsuitable for battle.
I recently sent my familiar into combat with a sphinx. Predictably, in retrospect, he died, and now I have no familiar. Now, I thought that he would be relatively safe, because I had buffed him up with a number of spells, including body of war, turning him into a warforged titan. (I was a 14th level wizard/incantatar in the Forgotten Realms.) But he died in the second round, as the sphinx proved to be a spellcaster and hit him with a flame strike. I hadn't (couldn't have) protected him against all possible attacks, and as he had lost his evasion ability when transmuted into a warforged titan, he failed his save and was incinerated.
All this came about because I really wanted to use that warforged titan miniature. It looked really cool right up to the point when my familiar was fried.
Familiars suck in battle.
-Ozmar the Less Experienced