You have to look at it the other way around.
avast! doesn't have so called "heuristics", yet it scored 39/40%. Then look at AVG which does have a heuristics checkbox somewhere in the settings. 40/43%. Thats no real difference. It's even worse if you look at McAfee, Norman or VBA32. McAfee also has heuristics checkbox while Norman is hyped over its Sandbox "heuristic" technology, yet it scored far far worse than avast! ? Or F-Secure that also has heuristics.
If you draw the line here, you can see that fact alone that program states that it has heuristics and offers a checkbox or sensitivity slider somewhere in the interface means exactly nothing. On the other hand program can have proactive features (eg heuristics) and doesn't mention that anywhere or doesn't give users the control for it. It's just there and works all the time. And tthats the case for avast!.
They don't hype that, they don't advertise it and they don't mention it. It just works.
Sure, i agree they have to move on and upgrade all this and they will in version 5. But for now it is as it is. avast! 5 is coming this summer anyway so until then i wouldn't really bother myself with all this stuff around heuristics.