I do think it has some important uses.
1. If corals or other inverts (especially many organisms of different species) are clearly suffering and one has ruled out common causes (nutrients too low, lighting or flow inappropriate, pests on them, etc.), then it can be a useful tool to help rule out some (not all) chemistry issues. I will note that even when folks have such issues with organisms thriving, ICP is most often not revealing of an explanation (based on folks posting at Reef2Reef). But it is worth doing.
2. Some elements can be tricky to measure at home (e.g., potassium, sulfate, most trace elements) and if folks are concerned about these, or just curious to check a baseline value, it can be useful. For folks not doing water changes, this reason can become more important.
3. Folks doing such tests need to understand the limitations of ICP, such as it saying nothing about the chemical forms of elements tested. If one had a value for, say, N, it would not be particularly useful without knowing if the actual chemical was N2, ammonia, nitrite, nitrate, cyanide, palytoxin, glycine, etc. I'm not sure why so many folks talking about, say, desirable levels for manganese, do not subject a value for Mn to the same scrutiny since it comes in many different chemical forms in seawater.
Despite intrasite similarities, Mn speciation was highly variable between our stations, emphasizing the diverse processes that impact Mn redox. Diel Mn measurements revealed that the cycling of Mn is also highly variable over time, even on time scales as short as hours. We observed a change of over 100 nM total Mn over 17 hrs and find that speciation changed drastically.
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JC015167
4. If one is going to dose supplements based on measured values, then for trace elements I'd choose ICP-MS, with the caveats mentioned in (3) above relating to what is dosed, and what target level or ranges one wants (including the rationale for those ranges). Matching natural seawater is not simple with an ion such as manganese that is both rapidly depleted in a reef tank, and varies greatly from place to place and in chemical forms in the ocean.
Overall, I'd do it at least once (I did do it, and did learn some things that were potentially useful, such as potassium not being depleted at all in my system.
