That's the advice of GM Nigel Davies, in the introduction to his 2005 book Play 1.e4 e5! (Everyman Chess). Davies notes that for many years he avoided 1...e5, but
[t]he turning point came when the former Soviet Champion Lev Psakhis once explained to me that an extensive grounding in the Ruy Lopez was essential if you want to develop your game. He added that in Russia it was said that the one failing of Lev Polugaevsky was that he never received this education" (page 5).
Well, maybe...I've read that Polugaevsky's failings (we're speaking relatively here - he was a strong GM throughout his career, generally among the elite and possibly a top 3-5 player in the late 70s) were primarily psychological in nature, but it's still an interesting comment - especially coming from a devoted Francophile like Psakhis. The Ruy is a very rich opening, demanding of both sides the ability to maneuver and to attack, and the ability to play all sorts of pawn structures and to handle action in the center and on both flanks. It's a difficult opening to master, but the skill set you can develop makes it worth the time invested. (Assuming, of course, one should invest time in chess!)
I hope my readers will find this food for thought, especially those who are dissatisfied with their current anti-1.e4 repertoires, and also those who are willing to endure a fairly steep learning curve in exchange for long-term improvement.