Still, it may be worth using Marketo if you're already comfortable with the editor and happy with your rendered emails.
Just wanted to add that we did this A LOT at past companies. For pretty much any Marketo asset program - whitepaper download or webinar invitation - that we built using program tokens and program cloning, we also created one version of the email to hand over to vendors. It was actually part of our standard program cloning process, so that version of the email was already done with no additional effort. The end result is an HTML document that - of course - you have to build without anything that can't work outside of Marketo. View-as-a-webpage e.g. or the above mentioned personalization. Also, our 3rd-party template email didn't include company headers and footers as they were not supposed to be used in that other environment.
But apart from that: Solid approach IMO.