The choice of cumulative distribution vs. time-dependent response times depends a lot on what you're trying to measure. The cumulative distribution is useful for showing the likely response time and its variation, but assumes a constant state. Your new plot style is useful for seeing how changes in loads affect the response time distribution (if you see double the hits, do you get longer response times and/or more variation in times?).
That's a really good point. In fact what you're describing is what the OP says people expect the first plot to show. Just because we (I?) can't perceive trends in the OP's second plot doesn't mean we couldn't if he increased the ab parameters.