That's my point exactly. Git wasn't created as a backup system, but why should it not be used as such? It's easy and efficient.

Bundling 5 plugins equals exactly 1 clone to install all 5 plugins. Otherwise it would be 5. It also equals 1 pull to update from master.

I don't think bundling implies they should be one product at all. For example, let's say I use WooCommerce, a user management plugin, a gallery plugin, and two other plugins. Putting those 5 plugins in one repo means you only have to manage 1 repo and can install all 5 in one go. And it will make it easier to update all 5 across the different projects you manage (assuming you consistently use the same plugins over multiple projects).