Escorts will stick to your rear flanks and shoot at ships that come near/block them from getting to you. They are very rigid in that role. As long as thats what you want/need, they are fantastic. If you want a ship to fly next to and focus fire with, not so much.
So: I would never use a normal frigate for an escort. They are much too reliant on speed/maneuvering for their defense and venting, so will die without doing much good. Specialty 'tough' frigates can do however: centurion, monitor, brawler all work pretty well, though with different focuses. The brawler's main weakness, its backside, is taken care of by it sticking to you like glue, so it is a good gun option. Monitor is all defense: its their to stall the enemy AI and just tank shots until you've dealt with whatever is in front and then turn to blow up the flankers. Centurion is a jack of all trades, but it has really nice stats and can mount a kinetic/he/ion cannon combo.
A nice tough destroyer that is designed to push enemies back rather than kill is good: this is one of the few roles that the Enforcer is actually good at! They make good companions for Dominators and Onslaughts as they are strong enough to push back frigates. Just be aware that their firepower and shields suuuuck, so something else will need to kill the flankers.
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Right answer, wrong reason. The answer is "no" because it's not worthwhile to waste even a frigate slot in your fleet on PD. Just ditch the PD entirely and you will be better off. "As much PD as you need for self defense" is always 0. You do not need PD. You have a shield, it's generally more efficient and doesn't cost you any OP.
This is a very fringe view that I do not
at all recommend following. MANY ships have exposed sections of their shields, and even omni shielded ships are frequently in situations where turning the shields to face the missiles is suicidal. Nearly every ship needs enough PD to handle stray pilums and salamanders coming from the back/sides - that doesn't mean PD in every slot, but it means some.
There are in general a few 'levels' of PD to shoot for, depending on what you want:
1) No PD. Player only, and a risky/annoying option at best. Ok for SO destroyers, but even then its annoying and of marginal benefit.
2) Light engine/side PD: enough to stop 2 Salamanders and stray Pilums. Enough for ships with good coverage shields. This is perfectly acceptable on most ships. Not effective against fighters, so have main/secondaries that are.
3) Anti-strike PD: enough to stop a barrage of 8 harpoons or 2 reapers or 3 wings of strike fighters from the front. This level of PD requires some investment, but means that when the ship is at high flux it is not vulnerable to HE strike. Very useful for fighting carriers, stations, and remnants: stations will spew out unlimited missiles (harpoons are nasty when unlimited), while Remnant destroyers often pack dual reapers and love to fire them - quite dangerous. This level is also enough to be decent anti-fighter in terms of DPS, especially when backed up by main/secondaries good in that role. With this level enemy carrier fleets are not a threat as long as your ships stay reasonably together.
4) Dominant PD: enough to stop nearly anything. This is hard to achieve and perhaps not even worth it, but Onslaught from the front/sides and Paragon all around can achieve it with an officer. Takes quite a few OP and probably a hullmod or two, but doing this makes the ship completely immune to all strike weapons, and usually immune to fighters just from the shear mass of flak/burst PD. Carrier fleets become trivial, including Astrals, as they have nothing that can hurt the battleship even when massed. Able to beat even 'cheese' fleets such as Reckless Falcon (P) swarms through sheer PD dominance.
Usually I go level 2 on individual ships, though I love making PD nets with midline ships that become level 3 when overlapping.