I also see no problem with using them...
But if it was me, I wouldn't do it. Unless I was really unable to get other primers and it was "use these magnum primers or no ammo at all", I would simply store them and go to a regular primer. Two reasons:
1) there is always the chance one day that you will need or even simply wish to try a magnum primer somewhere for some thing. With these on hand, you will not need to go anywhere to track them down -- you will already have them.
2) let's say you make a load with the magnum primer and it runs GREAT and you shoot TERRIFIC and it's every little thing you had ever hoped it would be!
NOW WHAT?! Now... you used 'em all up, the leftover primers. Now... are you going out shopping for -MORE- of the magnum primers, just for this awesome load? But what about all the other things you load that don't need a magnum primer? Re-work all of those loads?! ARRRGH!
Obviously, point#2 is illustrated with artistic license.
It is, however, how my thought process works. Frankly, point#1 is the one that I keep close to my heart. I
love the idea that I'm pretty close to being able to get started with "dang near ANYTHING!" if I get the idea that I want to try something... because I have so many options in the components and tools that I keep.
This is from the perspective of a hobbyist handloader (much like that guy above,
NICK), the type of bench jockey who isn't simply trying to spit out ammo for the next range trip... but as someone who gets as much from the art of crafting ammo as he does from shooting and owning the firearms that eat through it all.