I am including the following sentences as to provide some light into how the functionality worked under P3.

The normal scheduling of activities is for activities to be scheduled contiguous. Under the special case there are competing starts and finish constraints on an activity, under contiguous scheduling the activity will be delayed as to meet all constraints while under interruptible scheduling it will be scheduled intermittent but without disclosing the details of how each segment is assigned.  

Usually it is desirable to schedule the activities as a continuous operation as intermittent work is less efficient, stopping and re-starting of an activity might mean re-mobilization of crews, the inefficiencies created might mean the duration of the segments might have to be longer than the duration of the a continuous operation.

Another argument against the use of interruptible option was that it was applied across the board on all activities when interruptible is desired only on some occasions.

There are cases when it is desirable to schedule the activity on intermittent segments because of what is known as reverse logic. Under reverse logic if you split the activity it might reduce the total job duration or because some work will satrt earlier it might provide some protection to prevent the activity becoming critical or delaying other activities. In some cases it might be better to fully stretch the activity and reduce workload of resources. 

The elimination of the interruptible option prevents the application across the board of something that very frequently is not desirable. If it makes sense to split the activity into several segments it should be applied only when needed and under a controlled and transparent split where you would know the scheduled start/finish, duration and float of the individual segments.

In Spider Project we have a float value for Start Flex, with Start Flex you can know how much you can stretch the activity start. With the use of simple formula and a user defined field you can set activities of your choice to be modeled as elastic but never intermittent where you have no control.  If you need it to be intermittent split the activity and decide the duration of the splits and how resources will be assigned, good practice shall not allow shortcuts here.

I do not use P3 anymore nor have P6, therefore I cannot make a comparison but hope the above example helps.