I've soldered a lot of pipe in my house so far with no leaks, but can see why the threaded adaptor to pipe option might be considered. I thought of going that way too when I did mine. Why? I thought (which might be common among non-pro plumbers):
- hmmm, if I mess up the joint to the valve it's going to be a pain in the butt to undo it and if I solder to the adaptor I can always unscrew it, toss it and start over (always thinking the worst)
- threads seem to be so much more 'friendly' than soldering, only a wrench and tape is needed (though yes, you still need to solder on the pipe to the adaptor)
Without being an expert plumber, I like many others I suspect always look at every possible way of doing things and try to figure out the simplest yet most goof-proof way of doing something.
Every time I solder pipe I get nervous that I'm going to mess it up. I always take my time to do it as good as I can, have never had a leak. But when you don't do it that often, it's always "did I do it right?" "did I put in enough solder?"