The reason why Intrepids were not viewed as "combat" vessels in the Dominion war was the same reason that Sovereign class ships didn't fight in ANY skirmish, they were very advanced ships that were more valuable to preserve than use as cannon fodder.
I'm sorry but that sounds completely ridiculous. The Dominion War was supposed to be Star Trek's World War II, and if it were anywhere near as bloody as DS9 portrayed it, then Starfleet wouldn't hold back ships from fighting just because of a fancy computer core.
The real-world reason for the Sovereign not being shown on TV was because Rick Berman didn't want to use the design outside of the TNG films. I can't remember exactly why the Intrepid wasn't seen outside of Voyager (except for the one DS9 episode), but I think it had something to do with Berman wanting to save it for Voyager too.
An universe explanation: Maybe Voyager's sister ships were assigned elsewhere. The Dominion War didn't COMPLETELY stop the Federation's long-range exploratory and diplomatic functions. Another possibility is that they WERE in the war, but just not seen on-screen (DS9 always said there were more ships then were visible on-screen).
The Enterprise was regulated to more diplomatic operations, since the Federation diplomatic core was trying to end the war (this comes right from Insurrection). If there were other Sovereign-class ships they were probably either defending core worlds (Earth/Vulcan/Andoria) or on deep-space missions.
So you believe that the Federation attack fighters, Miranda, Oberth, Constellation and Danube all out class the Intrepid in battle?
Well we should count how many Federation ships that did not participate in the war had in each class: Sovereign, USS Enterprise and USS Sovereign. Intrepid: USS Belerephon, USS Voyager (lost in the delta Quadrant at time of Dominon War), and USS Intrepid. Prometheus: USS Prometheus.
1-2 of each class were present during the Dominion war. Did the number of vessels in each class factor into its participation in the war? I think so.
The Sovereign didn't fight in any dominion battle because... hmmm, well I don't really know why. Why on Earth would Starfleet not send the strongest class of Federation starship to fight in a war where every able bodied ship was needed (as you put it)? Yes I know that Berman wanted to keep that ship in the films, but they could have at least mentioned that the Enterprise or Sovereign was in battle X during the show.
So me personally, I would not measure battle ability in terms of participation in the Dominion war. The Intrepid's were not designed to take extreme punishement in combat as it is more an long term exploration vessel than warship, but it had more than enough firepower and shields to outclass the majority of vessels employed by the Federation during the conflict.