I prefer the trivial cat, because the < redirect puts the source in the middle of the pipe.

  cat foo.txt | bar | blah > out.log

  bar < foo.txt | blah > out.log

Furthermore, it means the first command isn't special, if I decide I want something else as the first command I just add it. Pure... concatenation. heh.

It's useful to know both ways, I suppose. But "don't use trivial cat" is just one of those self-perpetuating doctrines, there's no actual reason not to do things that way if you want.