This was actually changed for Spring 1.2; the AOP Alliance classes had to be linked in explicitly before.
The reasoning here is that the AOP Alliance interfaces are often used implicitly, even more so in Spring 2.0. We just wanted to avoid forcing people to worry about adding an aopalliance.jar just for implicit use of AOP, for example through a TransactionProxyFactoryBean or an <aop:config> definition that only refers to pre-built interceptors.
As we haven't heard of any problems with this up to now, we do not have any plans of changing this approach.
That said, I tend to agree that spring-aop.jar should probably not include the AOP Alliance classes anymore, since it's intended for use in a multi-jar environment anyway. This would mean that only the full spring.jar, provided as a convenience over the fine-grained jars, would include the AOP Alliance classes.