I've asked myself this several times. I keep looking for the "all-in-one" solution.
I haven't found one, yet.
Everything I need to do in regards to work related tasks can be taken care of on a tablet. However, the apps I need to use only runs on Android and iOS devices. So, the PlayBook has become just that, something I use to play games or stream music/videos. I can't use the "Business Tablet" for actual business...
As I type this on my laptop, it's sitting on my desk next to my desktop. I like the portability of my ThinkPad, and for me, there really is no replacement for a great physical keyboard (in the ThinkPad's case, mechanical) for typing. Also, I get a bigger display which makes viewing webpags and videos a lot nicer, and even though tablets meet or exceed my laptop's screen resolution, actual screen size is king when it comes to scrolling through lots of content.
A tablet is a nice portable media consumption device. It's great if I don't have to put in much input other than typing a web address or clicking on links. Laptop is more productive in that area. For instance, using Quicken on my laptop is a MUCH better experience than using it on a tablet. Even though there is the quicken app for Android and iOS, my fingers/hands/wrists hurt after trying to navigate through the details of my finances on the app, and even then, the app doesn't support all the features the PC software provides, which means I get a less detailed or very limited experience, which is usually the case with app versions of software. The app is great for VIEWING information, but for actually inputting, editing, and such, a laptop is way better in my opinion.
As far as social media like FB, Twitter, etc. they have evolved over the years to become more optimized for phones and tablets. I don't use FB or Twitter much, nor do I blog. If I did, again, the tablet would be nice for viewing content but if I were writing in my blog, I'd want a physical keyboard and mouse. The closest I can think of in this realm is I keep a personal journal using Evernote. I used Evernote on my 9900, and although the typing experience is the best by FAR on any phone or tablet, I would prefer a laptop or desktop keyboard. I tend to write pages worth of material in Evernote, so I prefer doing that on my laptop. Using the 9900 would be the minimum in my mind as far as typing an extensive entry. Tablet/touch screen phone? Forget about it. In fact, I even did some statistical research on my own Evernote journal entries. When I used my all touchscreen phone or tablet, I typed significantly less - I only wrote less than HALF of what I would normally write when I was using my laptop. So for me, an all touch device like a tablet, is less productive.
And yes, I could get a wireless keyboard to connect to my tablet. However, most wireless keyboards made for iPad or Android devices are too small to type extensively. My hands and wrists begin hurt after using a compact keyboard. And even if I did find a comfortable keyboard to connect to my tablet, I'd still be running the mobile version of software and not the full-fledged program (see my above Quicken example).
My desktop naturally has more processing power. I don't game, but every now and then (maybe once or twice a month) I'll play a game, however nothing graphic intense (Total War, SC2, Civilization, etc). I use my desktop for everything that I use my laptop for. For my needs, laptops and desktops are equal in regards to performance. Again, I'm not a heavy gamer, and I don't audio/video edit (at least not any more), so they perform the same. And especially with what AMD is doing with their Trinity APU, I can run the aforementioned games on medium to high settings an average of 30-40FPS and watch HD video on multiple screens without the need of a discrete video card. So if I really wanted to, I could eliminate my desktop and find a laptop with AMD Trinity A-10 APU and just use that. However, I will not give up my ThinkPad and unfortunately, ThinkPad's don't support AMD APUs. Hopefully they do in the future.
The only advantage I can see in having a desktop is that it's easier to expand for future needs. I can add lots more RAM if I need more multitasking or memory and should I need to render audio/video, and a discrete GPU as well. When I mixed/mastered and produced music, desktop was the only way to go. I couldn't fit all the necessary components required to render and play back music and the speed and quality I wanted in a laptop. Nowadays, there's apps for iPad and such to record and mix music, even DJ apps. I've used them, and can't stand them. One reason is that they're very toned down limited versions of big brother software programs (again, see my above Quicken example). And two, I can't stand navigating and controlling something that requires quickness and precision with just touch. I need to use a mouse, and even then that's pushing it - my recording/production rig consisted mainly of hardware so I didn't have to force myself to be creative at the same time messing with mouse clicks all night long on a virtual interface; it was less inspiring for me at least. So, I can see why your musician friend alternates between several devices. There really is no replacement for that magnitude of processing power. Then again, if all you need to do is record a couple of (low quality) tracks and just messing around or brainstorming/jamming away, then yeah I can see why something like an iPad would be good in that regard since you can take it everywhere you go and just plug n play as you get inspiration. But, even then, I'd rather use a portable laptop via audio interface break out box for that...
So to get back to the OP's question:
I don't know.
Hopefully, my experience has given you enough insight to make your own decision.
For me, it comes down to having AT LEAST two devices:
- A tablet that I can just slip in a bag or pocket and take with me to kill time or entertain me
- A robust laptop, that is powerful enough to replace a desktop, for productivity