One of the problems with this metric is it's easy to manipulate. Exchanges can easily lie about their book depth and/or people can put in lowball buy orders they have no intention of ever filling. To find out if those buy orders are real you would have to purchase them and all the orders above them, which would be very expensive and move the price a lot.

On the other hand, the instantaneous price of a coin is easy to verify. Everyone who buys or sells at the exchange can verify it, so exchanges can't easily lie about it.

Here's an interesting solution for ranking exchanges by their order book depth in a way that's more difficult to manipulate. Perhaps it could be adapted to rank coins. https://bitcoinity.org/markets/rank_explanation