Couple things here.
1) Building a tabular model and using that in the Designer (desktop) only works 1/2 way right now. You can connect to the Tabular model, but the relationships that you've built aren't recognized. This will apparently be resolved in the GA release on the 24th.
2) You don't need to download the AS connector for use in the Designer.
3) Are you planning on leveraging the Designer as the DataSet on the PowerBI site?
4) There is an AS Tabular direct connection from the PowerBI Site, and there is no additional modeling work needed.
- You can build your reports and dashboards on site
- Word of caution as it relates to the "roles" in SSAS. It only applies to the individual who is creating the reports / sharing dashboards. So if user A sees everything in the model, and shares a dashboard with User B (who should only see "x") - User B will see everything that User A does.
I haven't tested this out with the designer, but I would imagine the roles apply if the actual PIBX is passed around. But that really isn't a solution of "sharing a report"
- The Direct Connect to AS from the site does require the AS Connector Gateway.
There are a ton of different options, so any additional details would help steer you in the right direction...