Is it worth it for any reason?

That depends.

Dialga with Dragon-Type moves will likely be the best Dragon in the game.
Dialga with Steel-Type moves will likely be the best steel-type in the game.
Heatran with the optimal moves will be the best Fire-Type in the game.
Mamoswine with optimal moves will be the best Ice-type in the game.

However, we won’t have any immediate need for any of those types.
Dialga raids will need Ground and Fighting.
Palkia raids will need Fairy and Dragon.
Giratina raids will need Ice, Fairy, Dragon, Ghost, and Dark.
Heatran raids will need Ground, Fighting, and Water.
Cresselia and the Lake Trio will need Dark and Ghost.

Any of the new Dragons will be good against Palkia or Giratina. Garchomp could be good against Giratina, Palkia, and Dialga. Mamoswine could be good against Giratina or Dialga. Giratina will also be good against itself.

Moves are still up in the air, we already have great counters to all of the types needed, but the new Pokémon could definitely outclass what we currently have.

Plus there are the tier 3 and 4 raids that could require certain types.

Ask again when they are released and we'll tell you

If gym offence matters in your area (it certainly does in mine) and flying dragons remain common defenders, Mamoswine would be worth maxing out as their optimal counter.

With its typing and given good moves, could Heatran be the bulkiest counter to the Normal-type gym tanks (especially in sunny weather)?

Everything hinges on the movesets, but Dialga, Heatran and Mamoswine are all in contention to become top-tier meta attackers if they get good movesets. Dialga especially would become the best counter to multiple future legendaries making it incredibly high value.

That's like asking is it worth to buy a ferrari. And it doesn't make any sense.
If you have candy and dust go ahead. If you are poor and can't afford question is irrelevant.