The main advantage of micing a soloist or spotting an instrument in stereo is that you can integrate that sound easily in to the main stereo picture.
Spotting the person in stereo allows you to "steer" the image with panning so that even if you want the soloist, for example, to be on left side, you have some tricks to integrate that sound in to the overall stereo image. You can avoid a pasted on solo sound.
One way to do the above is to pan one of the soloist channels hard LFT. and the other soloist channel at 9 or 10 o'clock. Now the soloist appears on the LFT where you want her, but is "floating" on the left side and not obviously sticking out as can happen sometimes with a mono spot mic.