Why Legolas?

[Earniel]

Originally posted by Ithildin55
2. Legolas, as the King’s son, aptly represented the Elves of Thranduil’s realm who had, however inadvertently, allowed Gollum to escape; a situation that could have significantly endangered the success of Frodo’s mission. Legolas may also have born personal responsibility in the chain of events that led to Gollum’s escape.

I had forgotten about Gollum's escape. In that light Elrond may have added Legolas to the fellowship as a sort of letting him redeem the Mirkwood-elves for letting Gollum escape.

Or perhaps something a little more politically tinted like: "Okay, so you Mirkwood-elves let Gollum escape but we'll still trust you, see? We're sending your prince on a mission of great importance."