I agree that the iPad would definitely benefit from the addition of the missing iPhone stock apps. I totally get not seeing yourself using a certain app or feature or whatever. However, I've always been of the mindset that, hey, if someone else thinks they could really use some kind of addition, but I don't personally think it would be useful in my daily life, why argue against it? I mean, sure, it's often helpful to point out that doing something differently might be easier, or or that alternate apps/services exist that might be similar to what they're looking for. But, each user is different, and just because a certain requested feature might not be useful to some, it doesn't mean it's a bad idea or that it wouldn't be super useful to other people. Just my thoughts on the matter.
In my case, I have a MacBook Pro Retina 15, an iPad mini 4, and an iPhone 6S. Throughout my day I'm constantly switching back and forth between them, so I really notice little discrepancies between devices, and it bugs the crap out of me when I need to go into the other room to grab my iPhone to perform a simple task or look up some kind of data when I have my extremely capable iPad and MacBook Pro sitting right next to me. Sure, some might not find things like a stock Health app useful, but plenty of us would, and nobody would be forced to use an iPad Health app or anything. There are plenty of features on all my devices that I don't find useful, but I definitely wouldn't argue against having them and I think they should stay because I bet a lot of people use them.
Anyway, regarding privacy issues with the Health app, that is of course a very valid concern. However, I think it should be left up to the user as to whether or not they would prefer to cloud sync that data. Personally, I only use Health for workout data and step counts and that kind of stuff, and I find the stuff that's already stored and synced via iCloud to be way, way more sensitive than what's in my Health app. Emails, calendar data, photos, the location of my devices, even some notes contain much more sensitive data for me personally, and they're already accessible from iCloud. But, I know that's not the case for everyone, and Apple gives us the option of whether to sync most of that stuff in the cloud or keep it local to devices. For me, I'd choose to sync my Health data because the convenience in being able to access it from my other devices would far outweigh any of my personal concerns. But that's just me, and again, every user is different! So, more choices here, and the option to access your own data from your own other devices (if you want) would be awesome for those of us who really want it, but wouldn't really have any detrimental affect for those of us who wouldn't feel the need to use it.
In addition to a Health app, another app that I'd like to see on my iPad (and Mac) would be a standalone Phone app with access to voicemails. I love being able to accept a phone call from my iPad and Mac, but I get a lot of junk phone calls and therefore need to screen those. Not having access to voicemails makes taking phone calls from those devices a little less useful to me. Again, an perfect example of having to grab my smallest, least-used device when two other perfectly capable devices are sitting right in front of me. And I agree having a stock Calculator app would be nice. Sure, there are plenty of apps in the app store, but it'd save users the hassle of having to either purchase one or download a free one potentially laden with ads when Apple already has a perfectly good one they can use.
Anyway, sorry for the wall of text! This all just hit on something that I notice in some form pretty much every day, so I had a lot of thoughts on it!