It is harmful if all the Gnutella nodes do the same. Recall that you're normally seeing about 16384 hosts in your horizon.

If everyone requeries once every 180 seconds for just one query, it will make the query traffic be 91 queries/seconds.

This is about the average traffic one gets today.

Gtk-gnutella is already nice to allow retries every 600 seconds. Other Gnutella clients authorize only 1 requery every 45 minutes!