ok the nl function is just ultimate laziness it's two characters shorter than "\n" and four shorter than qq(\n). i like new lines in my HTML. It should really be &nl so i don't have to define it up top.
As for the concat, in this case it works same as , but if you use brackets

td(["bob "."dan","alice"])
[download]

to construct a row of td()'s it makes a big difference wheather you concat or commify. you get

 
<td>bob dan</td><td>alice</td>
[download]

So habitually i use . for things in one td cell. I doubt the overhead is noticible.