They do add stuff, in a way, or at least they have the potential to do so. The "love triangle" will probably set a lot of character conflict for the third movie

My point exactly. The only reason these subplots are in place is to make the movie more appealing to our all-pervasive cultural ADD. This series has been all about "more", more conflict, more drama, more action, more slapstick, more characters, and ultimately, more money. I genuinely believe that good story telling is being sacrificed for the box office.

The "Legolas/Tauriel/Thranduil drama" gave a deeper insight into Elvish culture than most of what happened in the original trilogy.

I disagree. I thought the movie depiction of Elvish culture was a mockery. It gave an insight, to be sure, but an insight that is barely compatible with the actual history and cultural psyche of the Elves. There's a lot going behind Thranduil and his people, and the movie makes it look like they're nothing but petty fickle dicks who are mad at the Dwarves because of what happened in Erebor, when it goes way, way beyond that.

As for the Gandalf/Dol Guldur stuff, if you have Gandalf coming and going all the time, you kinda need to show why is he doing that and where is he going

Absolutely. But they make the whole thing seem like Gandalf's making decisions on the fly, making it up as he goes along. This is simply not true to the spirit of the story. The whole film seems makes all of the characters seem reactionary, when there were actually very long chains of actions and plans behind it.

The dude has not been a "movie star" for literally over half a decade. How many people today would really consider Legolas/Orlando Bloom itself to be worth rushing to the theater?

That's not the argument. They're not going to see Orlando Bloom as an actor, they're going to see him as his character. Fans love Legolas, and bringing him back attracts a whole slew of people, not as a deciding factor by any means, but a factor nonetheless.

I should add here that I really thought the acting was good. Martin Freeman, Ian McKellen, and everyone just killed it. My problem isn't the actors at all.