Usually when we see legendary pokemon, we say "WOW!" or something like that.But the 4th gen legends here aren't as what we expected it to be.I mean COME ON! Compare Dialga and Kyogre.When I saw Dialga,there was really nothing special about it despite its trademark move "Roar of time".Seriously Dialga has too many connections on its legs,face and its body Unlike Kyogre who has a cool smooth body and plus its much stronger than Dialga.And to make Dialga even stronger you need to get the adamant orb unlike Kyogre where you don't need one.The same goes for Palkia as well.Whenever I see legends here,I get this "Huh?" feeling here.I didn't expect Giratina to have a long neck.And I think only non-legends here like Lucario and Gliscor are cool.Is it really worth catching Sinnoh legends? If it is, how come?
It all depends what you're looking for. If you're playing competitively I think you'll be pleasantly surprised with some of the legendaries, Arceus is known for it's extremely high stats and both Palkia and Dialga pack a serious punch. If you're just looking for cool Pokemon to use try getting Darkrai and Regigigas. They have unusual attack strategies and are pretty intimidating.
I think a better question would be, if you already own the fourth gen games, why would you not bother capturing them?
Arceus, Shaymin, and Darkrai are uncatchable without glitching/hacking or a Nintendo event.
And yes, when compared against anything that isn't legendary, they are very worth it. I sweeped the Elite Four once with just a Lvl 56 Dialga and plenty of Full Restores, Revives, and Elixirs.
The "uncatchable" legendaries aren't that hard to get through trading, especially since Nintendo released the Member Card in North America. Of course, you can always cheat.
Heatran is one of the most useful legendaries out there, especially when you consider it's actually not in the uber tier list and therefore still allowed for use in competitive battles.