Didn't the book just say they drove the necromancer out and later on, in LOTR, only did Gandalf say he only retreated back to Barad-dur?

Nope, in the next trilogy Necromancer will be driven out by Galadriel. (wud be epic)

Legolas in NOT in the original book and was added by Peter Jackson. Although logic did permit for this, I feel he was give WAY TOO much screen time... Also Gandalf's quest, again slightly overdone... And have to add Mr Peter Jackson's decision to stretch thing to three movies...

For a start, I think lots of parts of the storyline that were extended look like a rehash of the previous movies. Eg. Kili - Tauriel played out almost like Arwen - Frodo in the first movie. Morgul blade injury, saving at the last minute, look like an angel, bla bla bla. Their entire escapade adds nothing to both the story and the lore of LOTR. It only adds some sex appeal. And I think the whole point of this pointless exercise was probably a marketing decision to include one female to an all male case in order to add some sex appeal. I'm not saying Tauriel isn't a welcome change. She can be the rock that unhinges a landslide that leads the wood elves to be less isolationist. (Hence the 3rd movie and the LOTR trilogy which saw participation by the wood elves). It's also nice to have some nice eye candy to stare at in between the more than 2 hours of testosterone and orcs screen time. But it's just so sexist that a woman can't be included in the story without she being entangled in a love triangle. With a dwarf at that. Yes I'm racist. Evangeline Lily deserves better than a choice between a metrosexual and a a bleeding dwarf. And I mean that literally. They could've just added her, let her play her role and end it at that. Sadly, they just had to add all that unnecessary love triangle bollocks and that Arwen - Frodo rehash part.

I can understand the inclusion of the Necromancer storyline, since that is an integral part of the LOTR lore and explains why Gandalf had to disappear for large chunks of the quest. To join an assault of wizards and elves against the Necromancer (or something like that). But Gandalf getting caught, seeing the orc army and waiting to be saved just plays out too much like when Gandalf got caught by Saruman.

A lot of the plot is also contrived, with lots of coincidences and fake suspense. This being a sort of children's book and all. When the Bard goes to defend the town, the town master coincidentally wants to capture him. For fun. When the orcs come in to the town just a few minutes later, there are no townsfolk or guards. At all. When the gang leaves the elf kingdom, orcs coincidentally attack, which coincidentally aids their escape. When the gang approaches the entrance to Beorn's home, the bear coincidentally attacks. When Gandalf meets Radagast. The whole thing was so lame with contrived suspense sequences. I guess, realising that the whole movie is very long and talkative, PJ was afraid the movie would be too slow and boring. So he tried to make it suspenseful. He only succeeded in tiring me out before the end of the movie and making me annoyed. These contrivances also happen in the LOTR trilogy, but it all somehow seems more annoying here. Maybe because the novelty has worn off quite a bit. Or maybe it was done in smaller numbers. Or perhaps it was done in a less obvious manner. I don't know.