I got the impression from a French preview that there would be some kind of unique creature(s) over and above the regular summoning spells, but it was only a short reference in a babelfish translation, so may or may not be accurate.
In BD the act 1 summoning doll could equip a crossbow, so kept out of harm's way can be useful throughout the game, even without upgrading. I only used it for the occasional tough fight (and as a mule, of course), and only upgraded it at the end of the main game (using the DK's skill points to learn the summoning doll upgrade skills).
