I would put a stop to it, unless it was necessary. Plus I would be extremely curious. If there is something constantly using 15% of your WAN pipe with no explanation, that would be a huge cause of concern for me.
At work I have a 25 Mb fiber circuit, when I used to manage our firewall, if I did a speed test off hours or on the weekend and it returned anything less than 24 x 24, I took a look at the GUI immediately to see the IP addresses of the devices using bandwidth and try to make sense of it. We all have different levels at which we care about stuff or shrug it off, as someone with a passion for networking and IT in general, I am more anal about this then the average bear and do not rest until I have all of the answers I need. Or junk something that's causing me problems and just buy a new one