Could you elaborate on why? The companies I have worked for have pretty much all used domain.com for marketing and app.domain.com for the actual application. What's wrong with this approach?
If there’s any scope for a user to inject JavaScript, then potentially this gives a vector of attack against other internal things (e.g admin.domain.com, operations.domain.com etc)
Also, if for example the SaaS you’re running sends a lot of system emails that really shouldn’t end up in spam filters, you can’t afford to let things like marketing campaigns negatively influence your domain’s spam score.