I often leave small energy slots empty because I can't afford the OP, which I like to spend on fighters, offense, and shields. I guess more generally I don't see much point to PD, since shields stop missiles and ALSO stop ballistics and energy, unlike PD. Interceptors also stop both missiles and enemy fighters and frigates efficiently. And offense is necessary to end the battle. So once I've maxxed out my spark/thunder/xyphos wings, grabbed hardened shields and maxxed caps (along with front shield conversion, which you can't convince me isn't mandatory on every omni-shield ship), and slapped on enough long-range medium and large weapons (with vents) to bring killing power, I don't have anything left for the small mounts, filling which (as Hiruma Kai noted) would cost a minimum of 48 OP (68 with IPDAI) on an Odyssey.
It just means we have different play styles. I tend to use the Odyssey as a player flagship, and in Vanilla they're hard enough to come by I usually only have zero or one. While I might put Front Shield Conversion on an AI Odyssey, I'd never put it on a player run one. Omni shield lets you raise the shield where you need it, while front shields always start at the front end spread out from there. As a player, you can be much more aware of the positioning of ships, and avoid or deal with flanks appropriately. It allows you much greater active flux management.
With player fitting and combat skills, I'd tend to be running ITU, IPDAI, Hardened Shields and Resistant Flux Conduits as my essential hull mods. Loadout will typically be max Vents, 2 Plasma Cannon, 1 Locust, 12 PD Lasers, and 2 Longbows. Rest of the few spare OP into capacitors, or maybe I'll switch a PD or two and a few vents for Flux Distributors. I essentially run few to no extra capacitors on an Odyssey.
However, that is because with proper timing you can dash and vent in the face of enemy capitals and have your omni shield back up enough and in position to absorb more damage. If you keep repeating that kind of cycle as often as the ship system is up, because of the Odyssey's innately high vent capacity combined with Resistant Flux Conduits and the player venting skill, you can get some really short vent times which in many cases makes your capacity not matter except in the most extreme cases of incoming spike damage. Alternatively, Odyssey is very good at backing off under its own terms.
I find if I'm fighting fighters, I generally don't want 360 shields, as my hard flux drives up faster. I'd rather put the shield in the path of the HE bombers, and tank the kinetic/energy weapons from the interceptors for a few second on armor until the PD or Locust do their job. Interceptors and fighters surround your ship. Bombers come from the carrier side then retreat. Also, since I'm generally moving around the enemy fleet ball counter clockwise, friendly fighters simply don't have the range from their carriers to keep up with me, especially if I'm circling to hit Astrals or Moras in the back.
As for PD versus shields effectiveness, I'd rather take ~120 soft flux from my PD lasers firing and destroying a reaper instead of the 2,000 hard flux (or even 900 after hardened shields + skill) from the 4,000 high explosive damage impacting my shield. In a missile, fighter, or Doom mine heavy situation, I find sufficient PD will tend to outdo shield + capacitors in the long run. And dual plasma cannon + free sabots from Longbows on request is strong enough to take down any vanilla capital including <redacted>.
