For a description of the nature of nuclear weaponry, see a contemporary encyclopedia.
Usefulness of nuclear weapons
Although nuclear weapons are no longer the most feared devices to have been wrought by the hands of mankind, they are still useful and thus still in use. It is, for example, not at all uncommon to see 100 megaton warheads being lobbed at capital ships. In deep space, nuclear weapons have finally found a realm where they can be fully used in tactical military situations rather than (if only for political reasons) strategic ones.
Use against civilians
The use of nuclear weapons against inhabited civilian targets, like all massacres of civilians, is sufficiently abhorrent to the Rlaan that no major entity feels comfortable doing so without a very handy explanation, cover story, or assurance of no witnesses. Fringe groups are not constrained by such political concerns and have, on numerous occasions, attempted to detonate nuclear devices in heavily populated areas. They have met with both spectacular success and spectacular failure.
The teeming, technology driven, metropolitan planets of the Andolians have long been a prime target for Luddite nuclear strike, but Andolian countermeasures have foiled every attempt to hit a groundside installation on any developed Andolian world. The only successful nuclear attack by the Luddites on the Andolians ocurred on a small colony world, killing off much of the port population. However, this primarily served to give the Andolians a free hand to ignore political fallout from the Purists while pursuing a particularly aggressive counter-offensive, putting into play the assets of organizations in Purist and LIHW space known to have ties to the Luddites. The relative losses in both physical and political capital so severely favored the Andolians that the Luddites have since publicly renounced the practice of planetside use of Weapons of Mass Destruction, although they continue to use nuclear devices in attacks on space-based installations and craft.
Rather more successful was the assault launched by a radical subsect of the Crusade for Bob. These individuals carpet nuked their own port city, thereby killing all individuals participating in that year's pilgrimage to the "great emptiness" and effectively silencing five sects of Fundamentalist Agnosticism.
All of the major powers have ordinance interception systems, typically a combination of hierarchical sensor networks and defensive installations: orbital, airborne, and ground based beam and kinetic interceptor batteries. These work fairly well at intercepting nuclear attacks (among other things) against a planet, provided that orbital superiority has not been lost. During a prolonged assault, however, these systems rapidly degrade in efficacy as component installations are damaged or destroyed. The systems of minor entities and underdeveolped worlds, however, are much riper targets.
Nuclear Weapons in History
The Fraternal War
The largest recorded survived nuclear exchange (a handful of incidents, including the notable sectarian dispute in the "Church of Scientography Reborn, reformed" resulted in case studies in M.A.D.) occurred on the planet Ktah during the Lightbearer occupation. Upon realizing their imminent defeat during the Hoshino uprising, the Lightbearers detonated a series of nuclear devices that were responsible, in the short-term, for 400+ million Klk'k deaths, and, in the long term, for over a billion more. Had they had them available, the Lightbearers would likely have used much dirtier devices, salting the entire planet, and almost certainly rendering the Klk'k effectively extinct. However, the only devices they had brought with them were laser induced pure fusion devices tuned for beachheadding rather than for salting. Nevertheless, the resurrection of Ktah stalled the Andolian war effort far more than any Lightbearer defenses, allowing time for the Lightbearers to prepare. Absent this time, the war would have been even more lopsided and swift than it was.