The only reason I use v nails is that they allow me to glue and join frames very quickly. If I had to glue up frames before I inserted the v nails, I wouldn't use them. I think I can make a better argument for not using them than has so far been made for their use.
My training as a cabinet/furniture maker has left me with an abiding distrust of mechanical fasteners. There is not one joint in cabinet making that is improved with mechanical fasteners. In fact mechanical fasteners will hasten the destruction of most joints (probably, all). The only excuse for ever using them is speed.
While a glued miter joint isn't as strong as the wood itself as has been asserted, it's plenty strong enough for its application. Miter joints involve 45% end grain and end grain glue joints are very weak, but miter joints are still 45% long grain and long grain joints are stronger than the wood. Picture frame joints are not subjected to undue stress, nor should they be, and for their purpose, glue only joints are fine. I've got frames I joined 27 years ago that are as tight as the day I joined them and I imagine they'll still be tight 27 years from now, so experience suggests that a glue only joint is sufficient for a picture frame.
Sure the joint is going to be subjected to expansion and contraction, but the two mated pieces of wood in the joint are almost always from the same species and more often than not from the same trunk of the same tree. They will expand and contract together at the same rate but expand and contract they will.
If a frame joint has two v nails in separate positions, they will aggravate the consequences of the uneven wood movement by trying to stop it leading to a greater chance of joint failure than if they weren't used.
Frames are abused and joints do fail, as do joints in furniture, and having repaired both furniture and frames I can assert strongly that glue only joints are way easier to repair than joints with mechanical fasteners. In fact the presence of mechanical fasteners often makes invisible repairs impossible.
Glue joints in picture frames are strong enough for their purpose, they're less likely to fail and they're easier to repair in the absence of mechanical fasteners. As to the very unlikely event of a failing joint leading to damage to artwork, I might add my observation that joints rarely all fail at once.
If there is worry about the strength of glue only miter joints, there is a time honored framer's trick that will make the joint even stronger: glue two legs together and allow the glue to set up for a couple of minutes, pull the joint apart and reglue. All my comments are based on a well made joint with glue applied to both surfaces and on the joints being clamped firmly but not with so much force as to drive the glue out of the joint (a starved joint).