We've done cruise length pass (as PP stated includes scrubs), multiple times day passes (most time not even using all we bought) and once did cruise length and 3 days in not using once and not seeing would be used with what we had booked, actually talked the mgr to credited us back (we had other spa services booked). Doing the math on we typically use (cannot use day 1 for most the day during open house and till after dinner starts or the last day) if forgoing the scrubs, day passes virtually always come out cheaper for us. The only time they don't is when we make the facilities part of our daily ritual which we have done several cruises (the change room large showers roomier than the small split bath showers in-room and more space enjoyable after using the Rainforest saunas/showers/warming beds or human soup/hot tubs).
Also depending on how many day passes they sell prior to sailing and how many length of cruise passes they get a rush on during the open house selling frenzy in the spa (sort of reminds me of timeshare sales), the length of cruise passes may (and can) sell-out. This ensures those with day passes bought and the number of cruise length "potential" visitors do not overwhelm the facilities.