Most the salvageable wrecks likely have been non-functional as mechs and would be used for spare parts, especially for the Successor States who'd want to examine C3i or VSP Lasers, among other gear.
C3i isn't that big a deal given how long it's been around, although some of the systems integration, certain experimental weapons, and other things may be of more interest.
It still presents a bit of a boost at least at the small-unit level, especially for small recon or Fire Support lances due to the lack of a single Master unit to be headcapped. It's not the be-all, end-all that the Clan ERPPC was when it first showed up, but it's still not a bad trade with regards to the tonnage between it and regular C3.
I always thought the reason that Celestials were so deadly during the Jihad wasn't so much the fact they were Celestials, and more due to their VDNI-equipped pilots having cybernetics which boosted the Mechs' performance beyond that of your regular IS Mechjock, since they were designed to go after similarly deadly Clan Mechwarriors. Between the VDNI, their other implants, and some of the tech upgrades like VSP lasers, the Void Signature system, and Mimetic armor for their BA-equipped troops, they were Hell on wheels until the rest of the Sphere learned how to counter them.
The VSP lasers aren't that big an issue in some ways and, if poorly used, are even less efficient than normal pulse lasers. Void-Sig was very rare and not that large a factor overall. Mimetic armor is more useful, but has its drawbacks compared to normal stealth armor. The VDNI and other cybernetics were considerably larger factors in the danger of the MD.
Void Sig was rare, but quite useful - that's why Devlin Stone continues to manufacture some WoBbly mechs, IIRC, although he had the ejection system removed. The MD did deploy enough new tech with their units, IIRC, to make them useful in the field... which was part of why Liao took over production of units like the Eidolon.
Remember that NAIS had been playing with the X-Pulse lasers and similar systems for at least a decade before VSP's went out there - they don't fix everything, but the technology behind them could have been used to refine some of the programs NAIS was running right up until it got plastered. Even if Davion or the rest never deployed C3i-equipped systems, the outlooks and the possible design shortcuts that the WoB used in their gear would have been useful for possibly perfecting some of the systems that the rest of the Successor States were deploying in experimental form from the 3060's right up till the end of the Jihad.
But by themselves, the Celestials weren't special - which makes me wonder why mercs who have to pay for their own repairs would even consider them... especially given that in Devlin Stone-world (aka: post-Jihad Inner Sphere), mercs are looked upon with suspicion due to their making their living by waging war, on top of the way whole units went over to the Blakists and then engaged in war crimes.
Well, that and the WoBblie predisposition to throw around nukes and other WMD's if they felt they were losing...
That's not really specific to the Celestials.
No, but being willing to toss out nukes or bioweapons wholesale makes an enemy rather dangerous - and the fact that they did so helped stretch out the war against the WoB by years, especially as units or key industrial or logistical distribution points were destroyed or conquered.