It's certainly worth getting one for the pokedex entry.
I'd argue that it's worth getting more than one. We don't know what changes will happen to the game. Maybe you have to somehow trade in one each of the Regis to get a Regigigas, in which case we should have several. But since we don't know what we need, or what we will get going forward, get some. I mean, there might be some new challenge where you get to bring in one Pokemon, and it has to survive something challenging. The Regis would all do well in this kind of challenge, so have a few, just in case.
I say this as someone who has exactly one Registeel (but it's a 100%), and exactly one Latias. Should have gotten 2-3 Latias, but was busy. Will get a few more Registeel, and anticipate getting several Regirock. (Got 10 Regice, went 10/10 on catching them, a few have good IVs).
(And yes, it is tempting to 'retire' from Registeel with a perfect record, 1/1, 100%, but I'm going to catch a few more when I get the time.)