We try to avoid making the controls too complex, if an uncommon scenario can be achieved with an event handler, I would rather let the developers implement it this way, instead of adding a property to turn on/off the feature. However, if you share your use case, I could discuss it with the other developers and consider the implementation of such property.
If it is a matter of setting a single property, instead of attaching an event handler in the XAML, you could implement the functionality as an attached behavior, that will be used like this:
<telerikInput:RadComboBox local:ComboBoxExtensions.MaxLength="10" ... />
which is virtually the same as a property in RadComboBox. In this case you will set the handler in the PropertyChanged callback of the MaxLength attached property. Please, let me know if you need an example and I will provide you with one.
the Telerik team