Whether or not to use an own tool over the existing ones is a matter of several factors. Though I have to agree that useradd is the better idea, using solely existing tools is a question of purpose and may or may not be justified in this case. Marza's script may be doing more than we see from the snippet.

F.ex, I wrote a script for a site I worked at that dealt with the details of calling useradd for me; the same script also did a bunch of other things like add entries to a URL rewrite map, an ftpd configuration and several other steps.

Makeshifts last the longest.