At most, it might be able to show that you spend a certain amount of time within programming-related text-editors and that you're not spending time browsing the web. Assuming this is the case, it still doesn't prove that you're doing things effectively or that you're creating software that matches requirements.
Also, if it is possible to fake out RescueTime by writing a program that sends keystrokes (haven't investigated if RescueTime has put effort into preventing fraud like this) then a developer could just set it up on a VM and then use another VM to surf the web all day and be seen as a "good developer".
With that stated, it would still probably be a good filter today since to my knowledge there hasn't previously been an incentive to fake out RescueTime data.