I can think of a couple scenarios where it might come in handy to use only low priority interrupts.

A. If you need the first few code locations for initialization code that would exceed location 0x08, it would be nice to recover this space by forcing all interrupts to vector to 0x18 after initialization.

B. If you need just a few more words code space, this would free up room between the high/low vectors. GOTO somewhere at 0x00 with GOTO or BRA at 0x08 leaves several words of code space unused between the reset vector & low pri vector.

For B of course you could just make everything high priority, bump it up to 0x08, but you still have a few words not used from reset to 0x08.

So I guess in some extreme cases it might be nice to have the option.