Tales of the Sword Coast just doesn't add anything significant enough to Baldur's Gate to make it a must have.
Baldur's Gate: Tales of the Sword Coast doesn't add anything onto the main Baldur's Gate storyline. Instead you get a new town north of Baldur's Gate called 'Ulgoth's Beard', a new tower for puzzle fans and some quest related areas along with some minor areas that are just there for exploration purposes.
You can get several quests at Ulgoth's Beard, one being related to Durgoth's Tower (the puzzle mania dungeon), one related to a dungeon on a frozen island and one set on an uncharted island where you're seeking the wreckage of the legendary Balduran's ship; there may be more but if there are I haven't seen them. Two out of three of these quests are fairly large and one even has a narrative (not spoken by the same narrator as Baldur's Gate, oddly) unfortunately that is all TotSC adds; besides the bug fixes and new features that could have been added in a patch.
Tales of the Sword Coast isn't really an expansion that you must play to get the whole Baldur's Gate saga. It adds nothing to the main storyline and only plugs in a few new and unnecessary areas and features that are only patch material. If you have yet to play Baldur's Gate and track down the Original Saga bundle that comes with ToSC; by all means get it. If you already own Baldur's Gate and are debating whether or not you want to get the expansion don't bother. A few new areas and side quests doesn't justify having to spend money on this expansion: unless it's really cheap.