* Security: Yes, there's not too much argument that can be made for unquestioningly allowing arbitrary code execution on your computer. I think part of this will improve with browser enhancements, so long as they go more in the direction of sandboxing than in the direction of "Hey let's give Chrome access to everything in your computer right?!" Doing important stuff server-side is probably safer for users. That said, I'm bad at security. I just don't know enough about it, so I can't put up much of a fight.
* Bandwidth use: I'd actually be really interested to see some research on this. Needing to download a different version of jQuery/AngularJS/Backbone for every website you visit is certainly not particularly efficient, but I wonder how long you need to be on a website before they've sent you more HTML data than you would have had to deal with pulling down the Javascript and just pushing JSON data into it. For the mobile web at least, you win.
* Regarding power usage and wear and tear, I don't consider it a crime to make a computer do computing, which may be our main point of disagreement. If I've got a 2+ gHz processor and a few gigs of RAM, what else am I going to be using it for? Almost all of my apps are web-based, and I'd rather hear my fan rev up a little bit while Firefox renders a big graph instead of waiting for a server to generate a picture of one and just send it to me.