Well this is debatable and Im sure there are individuals that know more about this thank I do but here goes: Caffeine as you know is relatively safe but mildly addictive, aspirin is actually very good for you as it thins your blood and can prevent heart attacks. Ephedrine has been linked to cardiovascular problems and this is why it was yanked off the market. My personal take is that Ephedrine is still a relatively safe product but it wouldn't hurt to take holidays from time to time (it will also help its effectiveness over the long haul).
In terms of why people would take an ECA stack - burning fat/increases metabolism. Make no mistake, ephedrine is VERY effective at burning fat and giving you energy. Often people will take it in cutting phases and to get a good boost before going to the gym...hope this helps