I don't know what Shopify's view is on this but personally I think this is something that should be handled by external apps rather than the system it self. PDF generation is a completely different system than anything else within the Shopify core. It would add another level of code maintenance for them and a huge support requirement as although some tools are good, PDF customisation is still a complex task and beyond what most of the Shopify store owners would feel comfortable customising.
On top of that sending out emails with attachments raises the chances of being classified by spam and being rejected by corporate mail servers. So although very useful for B2B stores, most ecommerce stores are B2C and a normal email invoice is actual beneficial as when you grow you will start to really optimise the content and use systems that will dynamically insert cross sells and promotions in to these emails as they have incredible open rates.
