Anyone can equip the songstress dressphere with a bit of work, but why was Yuna chosen to be the one to deliver Lenne's feelings to Shuyin?
If Shuyin was indeed Tidus' real-life model, it makes perfect sense that as the lover of Tidus, Yuna's character would resonate somewhere deep with Lenne's: they're practically in love with two men of a similar soul. But there could be more than that. In FFX-2, most characters were portrayed with a determination to let go of their past. However, Yuna explicitly stated throughout her journey that she was still not ready to let Tidus become just a memory. Both Yuna and Shuyin are extremely alike in that sense: they don't let go of their pain easily. Lenne's spirit must have felt Yuna's resolve through the dressphere, or she wouldn't have entrusted her song of last will to Yuna - albeit subconsciously. Of course, Yuna's caring character was also at work: she simply wouldn't leave anyone in need, not to mention a woman as heart-stricken as herself.
Flipping the coin, I believe that Lenne was also doing Yuna a favor by leading her to Shuyin and Vegnagun. When Paine told Yuna that maybe it was about time they let go of their past, Yuna was clearly distraught -- in her usual subtle way. She was already starting to feel discouraged when the man in Gagazet's sphere was identified as someone other than Tidus, and Paine's words really made her think twice about traveling any further. However, when Yuna completed her quest to deliver Lenne's words to Shuyin, she finally made peace with her own decision from two years ago. If strong feelings between two individuals can live through a thousand years, why wouldn't she be content with a loving memory from two years ago and live with what she's got? As it turned out, Yuna was granted a happy ending for all she's done for her people, but without Shuyin and Lenne's story on the back of Spira's history, nobody would have come this far. As Yuna herself once said, everything is connected in this world.