Disclaimer: for tax reasons, I choose to spend a sizeable amount on photography purchases annually. I tend to buy based on the possibility of making photography a serious, high-end business, but don't make money currently on any of my work.
I've got a mix of Profoto lights (D1 500 Air, B1, B2) and I'm thinking ahead to future lighting purchases. I love the convenience of the B2 system and having far less weight hanging out on a grip arm (i.e. beauty dish overhead), but they're a bit weak on power and the modeling light is most certainly wimpy (9W LED, equal to a 50W incandescent bulb). I could put a D1 out on the arm, but that's a lot of weight. I could buy a D4 pack and one or more heads, but WOW that's expensive. (Yes, the Acute packs are far less expensive, but they don't support the Profoto Air system aside from basic triggering, and that's a no-go for me. The overall lack of digital control and "engineering degree" needed to work them just doesn't jive with what I've put together.)
I know that a pro-series pack/head system opens up other features and capabilities: compatibility with a few modifiers that don't work (or don't work right) with the lights I have now, higher power (ability to put 2400ws into one head), brighter modeling light (500W beats the 300W of the D1), shorter flash duration (in many but not all cases), ability to use RingFlash head, ability to use other specialty heads (albeit also expensive).
Any of you go with a pack/head for studio use along the lines of a D4? If so, what drove you in that direction?