Have you checked to see what treating your water would cost? Chemical water softeners are relatively cheap. This one[1] costs roughly $0.30 per load of laundry. If it doubled the wash lifetime of your shirts you're throwing away a ton of money by not doing it.
I've tried it. It's a combination of hard water causing greying and the fact that the hems on the t-shirts (usually the collars) end up curling after 7 or 8 washes. It's not a 100% gain, maybe 20-50% for substantial additional hassle.
T-shirts cost me ~$400/year presently. I live on a 4th floor walkup, my washer is on my second floor (yet another floor up), chemical water softeners are heavy, and shirts simply don't cost enough to jump through hoops of buying, carrying, and stocking another heavy supply fluid (already doing that for detergent and fabric softener).
I try not to add significant inconvenience to my life for things that will save me under $1000/year.
To put it in perspective, I spend more on bottled water than I do clothing.
[1] http://www.walmart.com/ip/Calgon-Water-Softener-1-qt/1342460...