I used two needlers vs. three HAC because the ship I have in mind is Phase Lance Eagle. For other Eagle loadouts, it will be two kinetics and one HE. For other ships, it depends on mount location and coverage. What I use varies on what I have available and what I need.
Buying that Heavy Needler over a cheaper weapon is an opportunity cost. It means five less OP for other things I get it instead of HAC. So if Heavy Needler has an effective 1 to 2 OP bargain or bundle deal over HAC, not to mention Heavy Needler is a high tier item that is harder to buy from military market (and not part of the lowtech/midline pack so harder to get blueprint for), I am okay with that.
The biggest pros of needler that is not damage or OP cost are either turn speed or accuracy. I guess a runner-up reason is the problems with heavy kinetics make heavy needler in a heavy mount a good partner with Mjolnir on non-Conquest ships, despite range mismatch (because Mjolnir is such a flux hog).
I think burst is a bit overrated, and burst is not always better than steady. At least bursts that are a stream of weak shots and not a single high-damage shot for superior armor damage (or shield overloads against AI) like a blaster or hellbore. As far as I am concerned, stream burst is equal or slightly better than steady, not enough to be worth more OP on its own.
With Heavy Autocannons easier to find, I use them until lategame when I can build as many needlers as I want.
So yes, for your phase Eagle, you chose Heavy Needlers over Heavy Autocannons. There is no real reason to use Heavy Autocannon over the Heavy Needler. It's not a choice when you don't have Heavy Needler available. You chose Heavy Needlers even to the extent of leaving a useful mount empty beecause Heavy Needlers are underpriced and more OP effective compared to heavy Autocannon. You don't get 5 OP less for other things, you pretty much get 5 inbuilt flux vents with the Heavy Needler, and you usually want as much flux vents as possible. You get them free with your weapon, enabling you to get even more flux dissipation for your dps.
But yes, Megas and Talar, burst fire is not always more desirable, but is generally more desirable. I didn't include its burst fire or accuracy for that matter into the worth of the Heavy Needler anyways. Heavy Autocannon is a burst fire weapon in a way anyways, but with less burst. Heavy Needler is underpriced regardless of it.
Here is what I think, if all characteristics stay the same. Railgun should be at least 8 OP. Heavy Needler should be 17 OP. Minor quibbles, but odds and ends and all that.
Light Needler Cost is reasonable, but it's just that the Railgun is just so much better, and has higher dps and accuracy and higher damage against armour and anytime you would be considering Light Needler, you would most likely be limited in small ballistic mounts.