That's a good reason; I didn't want you buying one because They said you needed it. ("They" told me a lot of stuff when I was starting out, that I later learned was bogus.)

You mentioned not knowing how the parks felt about you taking water from the stream. My own experience is they don't mind (but will issue a CYA warning about not doing it) - they will get serious about minding if there are known contaminants that can't be filtered out. For example, in rural Ohio, there are certain times of year when agricultural runoff can raise chemical levels; there are also parts of southeast Ohio where coal mining operations have permanently (or at least for my lifetime) polluted some streams with toxins in mine tailing runoff. So, check with the local park authorities. (It's a good way to break the ice with a ranger, who might then share all sorts of information with you.)