Two other 'secondary' skillsets that TMs can fill besides rigging: Face and Sniper.
The sniper role is obvious. Multiple IPs are most important for people in the thick of things. Snipers, on the other hand, are working from a long way off, and under cover. And there is a certain psychological effect that snipers have on people. Someone's head turns to mist, and hardened combat troops dive for cover. Period. The downside is that there are limited sniping opportunities in the interior of corporate compounds (though I'd carry a combat shotgun for closer work).
The Face role isn't something you'd think of off the bat, but since the Technomancer already focuses on mental stats, that means they have more points in CHA than others (especially since CHA serves as their biofeedback filter). Add on the right skills, and an emotitoy, or empathy software, and you've got a healthy dice pool.
As for TMs in the core book sucking? It all depends on how you play. I've played them straight out the core book, before SR4A came out, and did it well. Of course, I was playing as a hacker/rigger. The trick is to play the 'swiss army knife' angle to the hilt. You focus on one or two forms to put at 5, and get between 2 and 4 on the other ones you pick up. Then you thread as needed, and summon sprites. For instance, when breaking into a node, I may have a Crack sprite working to cover my trail and remove evidence of my intrusion while I focus on hacking the cameras, and my Machine sprite is riding the drones in with the team.
Now, hackers or riggers can be better at any one thing than the TM, but TMs can be very good at ANYTHING, given just a bit of preparation.