Tanky supports generally want stuff like locket or knights vow or even righteous glory to more directly influence teamfights, and besides that gold isn't exactly free. If a promoted minion is off pushing a sidelane, it's a carry who isn't otherwise pushing it, so if people are actually paying attention to minion waves and managing stuff properly it can be a roundabout way of a support 'stealing' cs.
Of course it does come with some added side lane pressure while the squad could be seiging mid or something, so it's not necessarily always a bad thing. I suspect zz'rot is a more attractive choice most of the time, where the portal can be put in a relatively safe place to do its thing, compared to banner just being a single unit who can be murdered as soon as it reaches the minion wave.
Against magic damage heavy teams it's not awful, but otherwise it tends to feel kinda lackluster.
That said though, theres a ton more active item cooldown in the game now with new runes, so I could see a world where banner/zz'rot become more prominent for the seige minded.