I would say that if lots of users are using tabs as placeholders in this way, and I personally know a lot including myself, then it's evidence of the bookmarking system being a bad or poorly designed solution for this problem. Bookmarks are treated as long term archives for links, and more trouble to manage, curate, and organize than is necessary for ephemeral sites you'd like to read later.

Some people mention "read it later" features, which I personally use. Safari has read lists, but I would argue these are also poorly understood and under utilized by most people.

For me, I believe a solution is something like "read it later", but lighter weight, easier to manage, and more integrated so that it doesn't just end up being another out of sight, forgotten backlog of things you wish you had read.