I also carry a thermometer. Although rather than buying a "stream thermometer", I'd suggest getting an inexpensive, instant read cooking thermometer similar to http://www.amazon.com/Thermometer-El...ng+thermometer.

I use my thermometer year round. In the winter and early spring, it's helpful for determining whether the stream is wamring up -- sometimes a couple degree stream temp increase will trigger feeding. It's also helpful to find warmer water locations such as near springs.

In the summer, it's useful to know when streams are getting too warm. The usual wisdom is that as stream temps get to 68 degrees or so, it's too warm to fish -- fish aren't as active and can be over stressed. With a stream thermometer, I can find cooler water -- near springs or in a smaller tributary -- or just decide to go home.

It's also useful for determining hatch times. As you noted, BWOs generally hatch at 45 degrees or better. In my area, Dark Hendricksons hatch at about 50 degrees, Light Hendricksons at about 58 degrees, and trico spinner falls generally happen at 68 degrees *air* temp.

You can also combine these uses. So for example, when most of the streams are running low 40s in spring, I might seek out and a smaller, mroe spring feed stream that is running a little warmer and has an earlier BWO hatch.