Great discussion topic!
For me, they have to bring something to the fight to make them worth the points, and they need to be easy to manage on the battlefield.
Dystopian Wars and Halo Fleet Battles do this pretty well, I think. In Dystopian Wars, you can attach escorts to capital-class ships (I *think* they have to be either large or massive--no escorts for cruisers) and they would link their fire in with ship they were escorting. In my early games of DW, I used them almost every time I faced a Japanese fleet, as they Japanese had a LOT of rockets in the earlier versions of the game (it was their "signature weapon", so to speak); so I would run escorts with my flagship for a boost in anti-aircraft fire, which could be used to knock down incoming rockets. The escort ships cost just less than a frigate, so not too expensive, but certainly not so cheap that you could throw them in without regard to the point total. Unfortunately the American (FSA) escorts had no primary or secondary weapons; all they did was boost AA and concussion charge fire to ward off air attacks, rockets, and torpedos. Some other nations, like the Prussians, had at least secondary weapons on their escorts which could link in with the larger ship to throw more dice.
Also, the French faction had a support cruiser that you could pair with large or massive-class ships. Same concept, just more expensive--but it did add more attack dice!
In Halo Fleet Battles, I think all ships at cruiser-class and higher have the ability to run as a standalone element or an escorted/defended element. Ships are deployed as "elements" in HFB, meaning you have one base with one or more ships stuck to it. So for instance, the UNSC could run a Marathon-class cruiser element with just the cruiser, or add up to three Paris-class frigates to the element (all on the same base as the Marathon) for more firepower and survivability. Some ships, like the UNSC Valiant-class superheavy cruiser, have options to run Paris frigates OR Halberd-class destroyers in the element, again giving boosts to firepower and survivability. Obviously you have to pay the difference in points. One of the biggest advantages, in my opinion, is that it is a clean solution--all the ships are on one base, so they all move at the same time (as opposed to Dystopian Wars, where you technically have to make measurements and move the escort ships separately).
Firestorm Armada has similar escort rules to Dystopian Wars (as it is essentially the space variant of that game), and some factions have the ability to pair cruisers with larger ships in the same squadron for increased offensive and defensive capabilities.
I think we could probably do a whole podcast segment on your question, though...