- Speed & Bandwidth
- Usability & Accessibility
- Platform Support
Speed & Bandwidth
We also tend to off-load more and more of the processing to the client, and if you use minimalistic (or just outdated) hardware, it's painfully slow.
Usability & Accessibility
Not all user interfaces should expressed in a dynamic fashion, and server-generated content might be perfectly acceptable in many cases. Plus, some people simply don't want this type of interfaces. You cannot please everybody, but sometimes you have the chance to and the duty to satisfy all your users alike.
Finally, some users have disabilities, and thou shalt not ignore them, ever!!!
The worst-case scenarios here, in my opinion, are government websites that try to "modernize" their UIs to appear more friendly to the public, but end up leaving behind a big chunk of their intended audience. Similarly, it's a pity when a university student cannot access his course's content: because he/she is blind and his screen-reader doesn't support the site, or because the site is so heavy and requires ad-hoc modern plug-ins that he/she doesn't get to install on that refurbished laptop bought on e-bay 2 years ago, or again because he/she goes back home to another country for the spring break and the local bandwidth constraints cannot cope with the payload of the site.
Not everybody lives in a perfect world.
However, there's no guarantee that all your users have the privilege of using modern browsers (either because of corporate constraints - which force us to support antediluvian browsers for no good reason, really - or other reasons which may or may not be valid). As mentioned by "Matthieu M." in the comments, you need to remember that a lot of people still use lower-quality hardware, and that not everybody uses the latest and coolest smartphone. As of today, there are still a significant portion of people using phones that have embedded browsers with limited support.
But, as I mentioned, things do get better in this area. But then you still need to remember the previous points about bandwidth limitations if you keep polling very regularly (or your users will enjoy a nice phone bill).
It's all very inter-related.
Your mileage may vary depending on your project.
Because trusting somebody to write a funny comic strip every morning and trusting somebody to run arbitrary Turing-complete code on my computer are two very different things.
I am not a web developer, and I have only a moderate understanding of the way the internet works. So this is an answer from a user.
My experience leads me to believe many sites are simply poorly coded, whether out of laziness or ignorance: When I would view a basically static web page, such as a Facebook page, my CPU usage would increase by something like 15%, and drastically more with multiple tabs. Eventually it got to the point where I would have to wait for a response after clicking a button or link and my CPU would overheat and lock up.
On many of these worst offenders (sites), nothing visible is changing and nothing interactive is happening. I could only suppose the site's code was constantly making excessive refreshes, polls, and endless loops.
This drove me to install NoScript to free my CPU usage and stop browsing from becoming a frustrating chore.
The other wonderful add-on I use is FlashBlock.
In the end you really never know where the danger lies (nobel web site infected on techspot.com). Many zero-day (and other) exploits use javascipt; closing this one avenue of attack feels like a step in the right direction.
My main reason is that it suppresses the most annoying ads. I'd rather not use AdBlock Plus, since that can affect revenue for the sites I visit (and I've used a site or two where the terms of service said I was not to disable ads). NoScript limits the potential obnoxiousness of ads, and I'm willing to live with the rest of them.
There's also the security consideration, and that's largely related to ads also, since any site that sells ads has to be considered potentially hostile.
Moreover, I don't necessarily know a site is dodgy before I visit it. Some people enjoy sending out links to sites, and aren't necessarily honest.
On a fast machine, with a modern browser, nobody in their right mind disables it all the time. Which is not to say there aren't plenty of very "security conscious" people and others without the funds, desire, or know-how to be running a modern browser on a fast computer... It was only recently that IE6 stopped being the most popular browser on the internet!