Does anyone know why Magento has never built in subject functionality? Was that a matter of risk (e.g. conflicting price rules), or simply a business decision, to leave this to external suppliers?
There may be many reasons for that. It may too time consuming or just not popular enough among Magento users or any other reason.
Anyway it's great that Magento is open source and you can get functionality it's missing either by customizing Magento or by installing an extension. If you need multiple coupons for a single order, you can check out this mod https://amasty.com/multiple-coupons.html