It's generally a better idea to go for one or the other - the great damage/healing via Alpha Strike/Meditate's great AP ratios gets really insane if you stack AP, and Yi's other strengths are from a more normal AD build, but building him hybrid can work. It isn't, as you said, the most optimal choice, but the added damage on his Q and added healing on Meditate is almost always useful, so it isn't the worst, either.