whereas ATPL theory would be sufficient for the IR also
True, but you'd have to get that IR within three years otherwise you'd have to sit the (IR relevant) exams again!!
From my perspective, knowing that I stand a snowball's chance in hell of getting an IR within three years, went for the CPL theory option. If I get to the stage of doing an instrument rating, I'd have to sit or resit the exams anyway!
And, given the number of people on the same CPL(H) course as me, I'm not the only one who thinks along these lines!
Cheers
Whirls