Niche for Regirock?

But even in those roles, it's still a lot more like Lugia and a lot less like Rayquaza. Check the DPS; with it's low attack and high defense, the TDO may be good, but the DPS should be poor.

Unless you have way too many people at the raid, DPS is king, so best use for something like this is as an anchor to finish the raid and save a few potions. Me, I prefer the glass cannons to the tanks for raiding - more reliable win, and more damage balls for me and my team, even if it costs a few revives and heals.

Finally, Smack Down/Stone Edge Ttar also has only a single weakness to grass, and should perform well vs. Ho-Oh. Granted, that's a special move, but if you've got that, it probably makes an attacking Regirock irrelevant.

no. why do people care about a meta for the Regis? they are tanks...

As a defensive legendary, Regirock's niches can only really come out when battling a gym.

Regirock with Rock Smash and Focus Blast is a strong Tyranitar counter, especially if the Tyranitar is running dark moves or Fire Blast. Because of its huge defensive stats, it will come out of a solo gym battle fight with less percent damage taken than a Machamp, despite taking 40% longer to complete the battle. However, if the opponent Tyranitar is running Rock type moves, Machamp will be much better, with Fighting resisting rock and Rock NOT resisting Rock.

For Regirock with Rock Throw and Stonge Edge, if you're soloing a battle, at the end, Regirock will lose the same percentage of lifepoints as a Smack Down / Stone Edge Tyranitar, and will do much better than the same setup on Golem. Thanks to being defensive, if you're pairing with others who are doing more dps than you, you will come out with less damage than them. It could also have typing advantage in some situations, such as a Pinsir that takes advantage of Tyranitar's dark typing.

Personally, I wouldn't recommend to use stardust to power up a Regirock for either of these reasons. I don't think these niches are very important.

It could preform well against Regigigas if all three are required to battle it

As a raid attacker, it wont be good, but for gym clean up, it could be a better alternative to tyranitar and more potion friendly, but thats about it