What are the pros/cons of adding synthetic tenors for PCA on a yield curve with with few observable rates, and using complicated interpolation?

In emerging markets, some yield curves have observable rates only at a limited set of tenors, for example: 5Y and 10Y, and no quotes between them. But even in developed markets we observe 7Y and 8Y swap rates, and when we need a discount factor for a date with no directly observable quote, for example, 7.5 years, then we need to choose some interpolation/extrapolation method. Decades ago, everyone just assumed that forwards were constant between nodes, but now there are numerous complicated meth