I don't think it's necessary to treat the Summon Orcs spell as containing a typo. Rather, it's a spell with more limited uses that should be judged against the two Quests that the Orc Shaman appears in. In Lair of the Orc Shaman, the benefit of the spell is Orc placement/movement. A pair of Heroes could quickly put the Orc Shaman at risk on his right flank, but it is unlikely a third Hero will have enough movement to attack him that early. Summon Orcs would then be useful to shore up the vulnerable right (top) flank with extra Orcs without compromising the Shaman's left flank. Also, the table makes it possible for a pair of Orcs to be caught behind the Heroes, making them pretty useless for saving their boss. Summon Orcs can put them in a more tactical position as a meat shield, allowing the Shaman more casting opportunities.
The Summon Orcs spell comes into its own during the following Quest, The Final Conflict. The Orc Shaman starts on the board and is free to move and open doors, thus revealing Orc-summoning possiblilities before the Heroes have a chance to stop him. Summoned Orcs could then be placed behind battle lines, where their attacks could do more harm. Also, getting two more Orcs into an otherwise bottlenecked room makes Sharpen Blades or Shield of Protection all the more useful.
Patroclus wrote:… and even then, the 4 goblins are more lethal than the 2 orcs.
While I'll agree with you that normally 4 Goblins are more dangerous than 2 Orcs, I'd also point out that the Orcs in the Lair of the Orc Shaman are elite bodyguard that have improved stats of Attack 4 and Defend 3. This makes them more lethal than even 4 Goblins. Check the
last post of the thread to see why (an Orc elite bodyguard is 13 XP).