"grep <pattern> <files>" is not the same as "cat <files> | grep <pattern>", in that the former will prefix lines with filenames if there is more than one input file. What you want instead is "grep -h <pattern> <files>".
The advantage of using cat, therefore, is the few seconds of laziness saved in not reading the manual.
The advantage of using cat, therefore, is the few seconds of laziness saved in not reading the manual.