goooood, yes, keep em coming.
Netflix, evernote do that now...
These are good examples of apps that would be useful on multiple devices, but these currently are not universal apps. They are separate apps that had to be re-developed for each target platform and device type. The major advantage of universal apps is that they can be developed once for all MS platforms. The time saved can be used to further enhance the app with new features which also target all MS platforms at once. The universal apps concept benefits both developers and the MS users that utilize their apps. The developers reduce their development efforts by avoiding duplication of work while at the same time reaching more customers. Users get to have a consistent and synchronized experience regardless of the device they are using.
how are people currently hamstrung ?
do you mean windows phone, iPhone, android , or all mobile operating systems?
think of some applications you would like to work with on the phone, laptop and PC ... are there many ?
You seem to be focused on a list of specific apps that would be used consistently across multiple devices. There are tons of examples, including some that you already mentioned yourself. You mentioned earlier that you use "video conferencing" as a tablet/phone task and "playing videos" as a PC task. I'm sure you can see that most people would consider those things equally useful on other devices though. For example, I use Skype on my desktop/Surface Pro far more than my phone, but I like that my contacts/settings are still synced to my phone since I do use it occasionally for Skype and it's great that I can use the same video conferencing solution on all my devices (even Xbox One). Same with OneNote, the whole Office suite really, a program called "My Media Center" that can interface with my HTPC, Reading List, Weave, and probably others.
However, I think what you are missing is that it's not *just* about specific apps that are frequently used on multiple devices. It is about the overall experience even for somewhat unusual situations. Greater overlap of functionality and seamless integration give the user more options in terms of how they go about accomplishing things with their devices.
Here is an obscure example that came up today for me. I order a lot of things online and I use an app called Package Tracker to keep up with the deliveries. The app runs on both WP and Windows 8/RT. I enter the tracking numbers into the app on whichever device I happen to be using and it syncs to all my other devices (including my Windows 8 + Media Center HTPC). I get notifications when something is delivered. Today I was on my desktop PC checking my work webmail. A coworker asked me for the tracking number of something I ordered for a project we are working on. Even though I originally entered the tracking number into the app on my phone, I was able to open the app on my PC, copy the tracking number, and paste it into my reply. It was as simple as it gets. If this had been a situation where the phone/tablet software is separate from desktop software it would have been an annoying experience. I would have had to go get my phone from the other room, open the app, copy the tracking number, paste it into an email, and send it to myself. Then I would have to go back to my PC, open my email to myself, copy the tracking number, and paste it into the reply to my coworker. Alternatively, I could have switched devices to my phone, opened the work webmail on my phone, found the coworker's email, and replied from there with the tracking number, but it takes a few steps to log in to my companies webmail so that probably would have taken longer. I also had a lot of typing to do in my reply and I wouldn't have wanted to do that on a phone keyboard. Either way, that is a lame experience compared to having direct access from the device I'm currently using via a universal app.
That specific example is not something I would consider a normal workflow, but similar situations arise far more often than you seem to think they do. Without universal apps, this integration is only possible if developers release their apps on multiple platforms. This is a lot of work for them and in many cases that means that some platforms won't have those apps and users will miss out on those integration benefits.