Hello, Paul! Our team has a couple reasons for using the Graph API directly.
The main one is that we are building different interactions to Microsft products not encompassed in the Integration Service!And since we now have projects using this API, we figured we could also extend the use for Sharepoint integration.
The other reason is we find that sometimes the integration service activities can feel a little inconsistent, and lack a couple features we would like, so we are implementing everything in house so we can have the most custom made activities for our needs.