With multiple clones, you would need to remember to update each clone regulary not only from remote, but also push/pull locally in case you want to compare your state with other branches.

A 'git checkout' on a large repository such as the Linux kernel may take a while (bound by I/O performance). Regulary switching between branches with many changed files becomes annoyingly slow.

If you keep multiple clones, you actually keep multiple copies of the full history, which seems like a waste of disk space (yes, deduplicating filesystems exist, but are rarely used).

git worktrees are also very useful if you have unfinished on a release branch. You do not have to make temporary commits or stashes you forget about, you can just leave the modified files as they are and switch to a different worktree to continue your work on another branch.