It makes them more similar to fighters, which is bad. It also encourages heavy all-in spam even more, because the more missiles you launch at once, the more will survive PD and return, which is also bad.
I think the similarity to fighters is only superficial, and only with the bombers at that. For example, a huge difference is you can assign fighters to guard your own ships instead of flying out to attack. There's 3 other fighter types besides bombers, and those 3 stay out indefinitely. Fighters are always restored after being destroyed, just need some time. So quadruple ordo, fighters still effective at the end of the fight. My suggestion will see those missiles run out if destroyed. Most fighters have a 4k range, missiles mostly have 2.5k or less. Bombers are slower, have significantly larger hit points, and hit significantly harder. Bomber "refire" rate is gated by distance to target and speed that the bomber travels, missiles can fire the next missile as soon as the refire rate is up, even if the previous missile is out. This frequently happens with Pilums. Lastly, there's a hullmod that lets any ship have fighters. I feel this is enough differences to prevent any strong feelings of sameness.
As for spam, well, as you noted, DEM already wants heavy spam anyways so that all the shots are taken advantage of. Otherwise as soft flux, it goes away. I feel this return reload idea might even reduce the want of fleet wide spam because fighters and other missile types can act as that cover combined with a less concerned about ammo, you can spam the firing them more often.
... which is also bad.
Why is missile spam bad? We're past the point where the best players stopped using PD because missiles aren't dangerous enough to redirect the OP and flux venting away from doing extra damage. I'd even go so far to say that it feels like it's detrimental to equip PD because the damage PD reduces from incoming missiles is less than the damage taken due from having a longer time to killing another ship due to the OP and flux going into that PD weapon.
And this isn't an attack, I genuinely feel that missiles should be dangerous enough to want PD on all your ships, on most small slots, and escorts with PD to handle any multi ship burst missile fire against one target, and if you don't have it, you need to strongly consider that ship a glass cannon.
If you feel otherwise I'm interested in the reasoning behind the opinion. Some space games don't have any missile PD.
They way you worded this reads like you're just looking at them in the refit screen and maybe trying out a few in the simulator, rather than using them in the campaign genuinely.
I've splash them into my fleets when they first came out and saw how shields effectively ignore them, particularly the larger ships. It was immediately apparent that DEM needed to meet some minimum threshold in quantity to be usable. They are particularly good against pirates that had no shields. They were good at overwhelming small craft in general like frigates and some destroyers, with the amounts that I had, then I ran out of ammo. I could already take out frigates and destroyers handily with the ships in question, while larger ships have enough dissipation to effectively ignore them.
I've watched videos, and don't see many people using them at all. I read the stories on these forums and reddit. Surprisingly lackluster interest for DEMs. Almost like they don't exist.
And while Gorgons have some love in the recent thread on general (they act like a 2/3rds strength phase lance, and like the phase lance, they're very dangerous to small ships and not much of a threat to larger ones), I've never seen or heard of someone using a Gazer (the graviton missile) seriously. It's less effective than a ship with a single graviton, and it's not just the question of ammo. I've made good use of graviton ships in suppression roles, but I can't seem to make the missiles do that. If you have a video showing good use them, I'd love to see it. AI tends to jump targets quickly, so the graviton missile ends usually ends up discharging at a ship that's no longer being fired at by other ships, where as the ship with a graviton jumps targets when other ships do. Lastly, the combined damage that it does (1000 kinetic softlux over 10 seconds) directly competes with a Sabot (200x5 kinetic in hardflux + 200x5 emp applied instantly). You can see where it loses badly here. It's only advantage is more ammo capacity and less likely to be shot down.
Dragonfire is something. Guidance, burst damage vs armor, the delay after getting to the target, and range vs reaper's ammo, burst fire rate, and availability in small slots. I'm not sold on the ammo counts yet, even after ammo bonuses, because some times, particularly later in the game, you are fighting against more than one fleet at a time, and the ammo causes you to run out. I'd rather see Dragonfire have more ammo, but a much larger refire delay. Or have reduced ammo with the chance to recover that ammo and fire it again.
If you compare this suggestion against omega missile weapons, it'll make DEM a poor man version of those weapons. Which is a cool dynamic.