The Philidor: Lopez counter-gambit is very rare and occurs in less than 1 in 1000 games. From the data, you can expect that White will have a 46.55% chance of winning, while Black should have a 53.45% chance of winning. Because of the fact that most players have probably never seen this opening and that White has such a low expected chance of winning, it may be well worth it to keep this variation in your back pocket. Since if you are Black, you should have a significant edge and will probably catch your opponent off guard.