Any type of tool or insurance or safeguard--and a water filter is all three--will be seen by some as necessary to secure and maintain, and by others as an unnecessary luxury, even an odd encumbrance.
Some carry weapons, others say, "I've been walking the mean streets for 47 years, and I've never been attacked."
Some have life insurance. Others say, "My brother-in-law has spent $30,000 on his life insurance over the past 35 years, and he's still alive, but working hard to be able to pay for things like life insurance, while I just live healthy and laugh at him...and don't worry about such things."
Some carry tools in their vans or RVs. Others say, "I've been on the road for more years than you've been alive, and any time I've needed a tool I've easily borrowed it from a stranger who stopped to help...or else I just waited for AAA or a mechanic to take care of it. Who needs to clutter up my space, or affect my gas use, with heavy wrenches?"
It seems to me that there's no absolute right and wrong on these issues. There are always arguments for, on the one hand, preparedness and independence, and, on the other hand, for simplicity and traveling light.
The way I try to make such decisions is to weigh (literally and figuratively) the upsides and downsides of carrying/having a particular thing.
I take into account its possible benefits, balanced against:
a) Money cost (initial and ongoing)
b) Space
c) Weight
d) Maintenance requirements
e) Need to protect it/Worry about it being stolen
f) Misc.
In the case of a pocket-sized water filter that can be bought for $20-$30, my own decision is to keep several.
I have several kinds, but I recently found the Sawyer mini water filtration system, which costs about $20, and filters down to 0.1 microns (even the Katadyn Pocket pump filter was 0.2 microns), and can last up to 100,000 gallons. And it's tiny.
http://www.amazon.com/gp/product/B00FBGCLSC/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1
But I think much of what we carry and don't carry with us, or even own, is a matter not of logic, but of personal comfort and personal preference (to which we of course add logic afterwards): We're all cautious and prepared when it comes to some things, or some areas of life, and remarkably un-cautious and unprepared when it comes to other things.
Or so it seems to me.