That was my point earlier, it doesn't work on everything. It works on everything directly related to elements though (old elemental staves are an odd example, since they're crafted and not sure if that should work on crafted items), sea gorgets, obis, sachets, belts and magical trial staves.
One could argue that if it's implemented for trial staves it should work on other trial weapons too, however I think there's still a clear difference between elemental staves and other elemental weapons, namely that they affect an element directly (elemental cast time, damage, accuracy), whereas other trial weapons affect properties related to that element (STR, Evasion, PDT, etc.). I think an exception for magical staves shouldn't be too exceptional, seeing how stave trials are an exception all by themselves. These trials only exist for staves in the first place, all other weapons either have two elemental trial paths (for each element) or three, if a pet job can use it (Axe, Hand-to-Hand, Staff). Staves get another two paths per element, one for magical accuracy and one for magical damage. Since this is an exception to the usual pattern in the first place, I don't think it would be much of a problem to make another exception to offer a combined version for staves.
Another difference to why it should work for elemental trial staves but not for other elemental trial weapons is that the effects don't stack. As your example shows, if you combine the old elemental staves you could enjoy lots of boni from having that "Rainbow staff" equipped. That's not the case for these, since it doesn't matter that you have the thunder and ice damage bonus in one staff, since you can't use both at the same time anyway.
In short, yes, it shouldn't work for everything. But for every series of items that has mutually exclusive uses, it would still be a great solution without being in any way game breaking.