I did so twice; once worked beautifully well (Eliot/Parker/Hardison) and one war a spectacular mess (Kathy/Nick).

Because it's 50/50, I'm not sure if I do like it, though having RP partners that you can get deep with right off the bat was great. Perhaps it's more "I like bringing in people with an established relationship already, but I don't like bringing in characters who WILL get together with someone later."

Give me an established pairing or give me freedom, I cannot abide the middle ground of expectation.