The thing I tend to tell myself when I feel the temptation (yeah, slippery slope I know) is that if nobody can tell the difference, it didn't happen.
I have taken some cuttings from public landscaping around here, and actually some of the plants in the park come from them. In that way the plants that may overflow one space end up populating another, through the magic of pruners, and nobody pays the price. Almost all my landscape gardening takes place in public spaces, given where we live.
I do draw the line at institutional gardens like arboretums and bot gardens though. Personally, not prescriptionally.
Partly because of my emotional investment in a project along those lines. Also out of respect for the ideas of variety and compatibility, and somebody pursuing them to their unpredictable conclusion.
A few years ago I picked up a couple seed pods from an aloe tree in an arboretum and felt guilty about it until I decided they were being trampled anyway, and pushed the rest of the inflorescence out of the way of oncoming feet.
Those young seedlings are about to flower for the first time in the park this winter and so the cycle of life will continue.
There is an important aspect of bot gardens related to public education and public access to plants. And ideally they would have a young version of everything in the garden available for purchase in their greenhouse, more for this purpose (related to their mission) than the earning of cash (related to their survival). But there is never enough space or expertise or volunteer hours to make that happen, so it's inevitably a patchwork affair.
