Well , not really worthless in that sense my friend, since i understand what may led you to think it as such, Since Aragen sees neutral to everyone in regards on how to react, and of course every other deity may show neutral to your Aragenite cleric, and same would be for the non-good aligned people, but there is one thing that divine relationship can still do in this very case.
Aragenite divine relationship can tell the cleric when other being is dedicated to the very cause as himself (serving Aragen), this is if cast upon other member of the faith it will show you that the subject worships Aragen as well, this of course making no diference upon the actions the cleric should take towards this subject but letting him know that the subject persues ( in his own way ) the very same goals of serving the lord of Knowledge. This very fact makes this spell letting the cleric to know valuable information in regards for example in whom to trust the preservation of knowledge, or to maybe exchange notes expecting some level of objectivity . // this is at least on how i've seen it work, but to the extend on how i've seen the spell work. . now some field testing would be good.
I understand as i said that it is very much limited in comparison to the utility that is for eveyother gods cleric compared to the Aragenite, but there is still this diference that from the Rp stand point, makes a huge diference to be called worthless.
Just my 2 cents.
// Edit : after some testing, As you could see on the attached picture, It actually gives you information on whom worships Aragen, tested this with a PC casting it on an NPC with a deity field filled ( mechanically ), and with my avatar upon the PC