"Wrong" is harsh, I agree, but I do wonder about folks that have so many tabs open.
Because browsers either don't retain the state when returning to a page with the back button or simply refresh the page and all its resources anyway, which makes reading documentation with several links in it that you need to investigate a laborous experience. Tabs on the other hand refresh when loaded, and on various programmatically triggered events, but not simply because you choose to view them at a particular moment.
A page in a tab that is already loaded will come into view quickly, and they usually retain state information like where you've scrolled to when you return them, which the broken back functionalilty unanimousely fails to do.
Isn't that what bookmarks are for?
No, bookmarks are for retrieving a specific page, or a pre-chosen place in the page via anchor tags. They are neither intended nor expected, nor capable of storing the page state at the time you viewed the page.
You're obviously not flipping between 100+ pages in the course of a single /task/.
Hahah, sure, some tasks don't require that. Other tasks require a lot of tabs open to different pieces of documention that all interact. I don't regularly find myself using 100 tabs for a single task, but I do find myself flipping between two or three tasks where I need six or so disparate pieces of information to be quickly available.
Is that frequency worth the resource load of having the page loaded vs loading the page on demand?
Exactly the opposite. Having the page sitting in a tab reduces the frequency of loading the page and can also be used to push that loading into the background while the user reads other material on the original page. Further, even in the case where pages in tabs have scripts running that tax the machine a bit, it's not the computer's time that's valuable, it's the user's time that's important, so a user will keep tabs open that need to be referred to later as long as they don't impact other pages or applications on the machine.
Is it genuinely faster to find a tiny little tab in a - presumably - rather squashed cluster of 100+ than it is to find a bookmark on a menu and have it load or am I envisioning what 100+ tabs looks like completely, heh, well, "wrong", because I don't do it myself?
It's self-limiting. A user with a good contextual memory might just just jump to the right spot, other users will only be able to do that with fewer tabs, and they will therefore use fewer tabs. You use as many as you can hold in your memory at once. Also, you don't necessarily keep them all in the same browser window. In Chrome, you can have sets of tabs open in separate windows, grouped by whatever criteria you choose. In firefox, you can have tab-groups that hide sets of tabs away entirely until you want to call them forth.