I was discussing this with Appocomaster earlier, and I'm afraid I don't see any good reasons not to make a planet's alliance tags public to paid planets in galaxy. I'd argue that it should go even further and make them entirely public, despite the increased risk of incoming this poses to un-allied planets.
On a related issue, I don't see any good reasons to not make alliance value public.
In my opinion there's too much for command members to do already, and forcing them to spend heavy amounts of time keeping intel up-to-date is pointless and favors activity over skill (after all, there is pitiful little skill involved in intel these days*).
If you can see any good reasons against either one, the other or both, please help educate me
*Most intel nowadays is planet based, and garnered by sifting through JGPs, asking members to squeel on their galmates etc.