I've heard of people who train their bodies to work better burning fat on long runs by running without a sports drink. However, the fat burning metabolic system produces energy at a much slower rate than the sugar burning one. This is where 'the wall' cones from.
The suggestion above to use electrolyte drink is a good one. You could also consider supplementing with salt capsules. Thermolyte and S-Caps are two brands I know.
For sufficiently long runs in sufficiently warm weather you definitely want to do this. As you sweat and as your muscles burn you will lose sodium, potassium, magnesium and calcium.
You need sodium and potassium to be balanced as you sweat them out. The concentration in your blood needs to be within a narrow range else you run the risk of such deadly conditions as hyponatremia.
Calcium and magnesium are used by your muscles to signal contractions. If you run out then you will cramp.
This is why electrolyte drinks and salt tabs have this 4 minerals. The magnesium and calcium will be a much smaller amount than the other two.
When you use a salt tab be very sure to take it with lots of water. They are concentrated enough that if they dissolve by themselves in your stomach then your stomach will detect this and you will throw up. I've done this twice!
Karl King's water/electrolyte balance table is a very valuable resource to have on hand because it helps tell the difference between a number of conditions and what action to take.
Statement of the Second International Exercise-Associated Hyponatremia Consensus Development Conference, New Zealand, 2007.
is a great source for the raw science.