[q]They were actually more expensive to produce than a similary configured CA hull would be, more vulnerable in open space, and fairly rare. If would be cool to have them if they could be employed with limits. [/q]
The problem is that the way they were implemented in SFB, they are not that expensive and not that vulnerable. Especially the war variants. If I recall correctly, the Klingon tugs actually had substantial firepower, even without pods. In game terms, the limitations of the tug combos were not really that evident. Why build big CVA's if you could just use CVA pods on your tugs? Put more money into the escorts. (essentially what the US navy does). That way, losing a CVA would not hurt as much.
My thought on Tugs goes back to the Franz Joseph Design manual. The idea of Tugs is to provide a fast freighter capability. . .presumably standard freighters are too slow for certain tasks. That was also an idea behind the Scout. A key secondary mission for Scouts was "VIP Transport".. . .again, presumably standard liners and yachts are too slow. . .
. . .in game terms, this could be achieved by simply eliminating the funky pods. And I think a cargo tug could make an interesting target for many scenarios.
. . .I also don't think freighters are appropriate for all races. Feds => sure. The federation is way spread out. The Hydrans Kingdom is small. . .and they probably shouldn't be building such big freighters at the expense of first line warships.
Klinks might be a special case, but I also can't see the Klingons expending resources on fast freighters. Fast troop ships, maybe. But keep in mind that the D7 and D6 have gobs of marines and transporters. . to serve the troopship function (just like an Imperial Star Destroyer).