I could say the exact same thing about a laptop; for my needs, I have to have a desktop at home acting as a server for storing/streaming media (Plex) and stuff like that. I have always remoted into my desktop, be it from my rMBP or now from my iPP, to do things like manage downloads or big file collections. So swapping the laptop with an iPP isn't a big deal for me.
Conversely, there are folks for whom just having a laptop and/or an iPad (or even a smartphone, for that matter) is enough computing power for their needs. Different strokes, man.
I'm not talking about computing power or hyper-niche uses here. A laptop can exist on it's own perfectly fine. It can download from anything and can connect to nearly anything. Importantly, you can back it up to a USB harddrive, or make online backups to any number of competing services that integrate and work super well on an OS-level and offer a plethora of different features (Apple's Time Machine, CrashPlan, Carbonite, and dozens if not hundreds of others). iOS has only two backup pathways - (1) syncing with another computer running iTunes or (2) Apple iCloud - and neither of those two ways are really ideal since they aren't designed to be backup solutions primarily, rather file syncing solutions. Files have been lost in iCloud, files have been corrupted during an iTunes sync. In both these cases, a user would be wise to have a real backup as well (one of the ones I listed above). Any other cloud service available on iOS are nothing more than a glorified viewers and uploaders - none offer the functionality of actual backups.
This backup ability is key - as without a proper backup, no one can or should really trust the device to be a primary device. Photographers can't trust their photos to it, writers can't trust their documents to it, etc. Nearly everything we do is creating or modifying or adding to or otherwise working with some kind of content. Why should people trust a device to serve as their "primary computer" that can't even keep their files properly backed up?
Even the "regular users who only browse and email" that iOS-as-a-primary-device proponents always point to are likely to have lots of photos of friends, children, pets, vacations, and the various other miscellaneous documents everyone keeps, and that they would be devastated to lose. How does someone keep a 20GB photo library in a way they will be relatively sure it will not disappear suddenly using only an iOS device? Pay for iCloud $1/month in perpetuity for life? That life-sentence seems totally unreasonable, and isn't even a sure thing as plenty of people on this forum talk about files lost in iCloud. Or they sync it to their actual primary computer with iTunes, and then back it up to a USB drive or online backup service. Certainly, you can't just keep it on iOS and assume nothing will ever happen to it.
This is why iOS pretty much must have a companion laptop or computer, and cannot be a primary device for anyone as it exists today.