Unless things have changed in recent versions, the real issue is that PBP doesn't use interrupts in the conventional sense. To get around some of the issues Henrik pointed out, PBP uses a polled method which can drastically effect the latency, or interrupt response time.

Some snippets from the manual:
When an interrupt occurs, it is flagged. As soon as the current PICBASIC PRO statement’s execution is complete, the program jumps to the BASIC interrupt handler at Label.

Since PICBASIC PRO statements are not re-entrant (i.e. you cannot execute another PICBASIC PRO statement while one is being executed), there can be considerable latency before the interrupt routine is entered.

PBP will not enter the BASIC interrupt handler until it has finished executing the current statement. If the statement is a PAUSE or SERIN, it could be quite a while before the interrupt is acknowledged.
So, basically if you're using the ON INTERRUPT method, PBP polls the IF between PICBASIC statements, not at the uC instruction level. Depending on what you're doing, this can make them useless.