With Flower Veil Virizion can switch with impunity into Rotom-W and SubToxic Gliscor, which is great. However, the biggest boon is the ability to get past Chansey with a CM set, which can't Toxic Virizion anymore. Then, on the last slot, Virizion can use Hidden Power Psychic to deal with Mega Venusaur, which does little damage to +2 Virizion and gets 2HKOed back. Unaware Clefable can stop Virizion, but not all stall teams use it and even if they do, Clefable is not the hardest Pokemon to wear down from my experience, due to its reliance to Wish + Protect for healing, which is easy to take advantage of.
Also, SD Flower Veil Virizion with Close Combat / Stone Edge / Leaf Blade is not half bad against offensive teams, as long as you have a Pursuit user to get rid of Aegislash and Latios. Flower Veil not only gives switch-in chances against Rotom-W, but it also protects from Landorus-T's, Gyarados's, and Mawile's Intimidate, and most importantly, Thundurus's Thunder Wave, which can do little to Virizion otherwise. At +2, Virizion OHKOes most Pokemon found on offensive teams after SR (outside of the already mentioned Aegislash and Latios), and can OHKO most Pokemon that outspeed it or have priority to KO it as they try to switch in, such as Greninja, Talonflame, Scarf Excadrill, and Mega Pinsir. And of course, being able to revenge kill +6 Azumarill and +2 Bisharp is great, as well as check Pokemon such as Excadrill, Thundurus, Landorus, and Landorus-T.
Oh and Virizion's special bulk is so good that it can tank even one LO Ice Beam from Greninja and OHKO back (96.2% max) without any HP investment. Virizion can even take the role of an offensive tank that can check dangerous threats thanks to Synthesis being much more reliable in 6th gen.
tl;dr don't hate on Virizion, it's actually very decent with Flower Veil.