"Firstly, cognitively you can't keep 200 items in the stack"
(TL;DR at the end)
I'm guilty of keeping hundreds of pages open at a time. Right now I have 4 windows with 214, 180, 158 and 102 tabs. In my defence, I'll say first that it's not a stack. It's more like a serialised/flattened tree (or actually, a forest). When I middle-click a link, the new tab opens next to the referring page so the flattened tree structure is maintained no matter how many pages I open.
Most of the pages that I have open relate to some particular search topic that I've been interested in following up on. The easiest way to do that is to speculatively click on a bunch of promising-looking results from a search engine, scan some pages and then either refine the search or drill deeper within existing search results or the sites that I've already opened.
I don't think that using bookmarks is a very good way for dealing with this kind of ephemeral collection of pages, although if the browser had a feature to bookmark (or pop out into a separate windows) a range of tabs, I would definitely use that. Instead, If I want to jump back into a particular search tree, I just go to the address bar and type some relevant keyword and use the "switch to tab" feature to find where I was when I went off to do something else. This is much easier and less work than using bookmarks or trawling through the history (which is basically unusable in Firefox) to try to recover the state of my search trees.
Every so often I do a sweep of open pages, starting from the most recent (rightmost) tabs. It's usually easy to spot a range of tabs and delete them all (individually; again, a "delete range" function would be brilliant) without needing to scan the contents. If I remember that there was something in that tree that I might want to come back to again, I'll find the best links and note them in some way (in a bookmark folder or in a wiki that I use for note-taking) and then close all the tabs. If I know that I haven't finished some search, I'll skip that range and deal with it in a second sweep.
So anyway, the TL;DR: if you have enough RAM to be able to keep loads of tabs open, it makes for a very easy and lazy way of keeping on top of tons of disparate islands or pockets of information that you're interested in. You probably want to scale back on doing this sort of thing on a work machine (find some other way of reminding yourself of things to check out later) but it's nice in the comfort of your own home.