Should we seperate the CMS interface from the rest of the website?

Our company makes websites. Our website has a loggin area for people who already have websites with us. The clients can log in to have access to a user interface for modifying content on their website.

My question is; would it be benficial to keep the "user interface" on the same domain as our company website?

Will traffic to our homepage from "already clients" increase page rank?


e.i.

www.mycompany.com/admin/ for CMS

v.s. on a subdomain
admin.mycompany.com for interface

Pros that I see for same site:
1) more traffic to our homepage by users that want to log in. Increasing page rank? <-- with a question mark
2) Will only need to program log in stuff once on one website.

Cons that I see for same site:
1) Analyics will be jumbled by users that what to log in v.s. new customers visiting the site
2) When we do a full publish of changes on our company site, users will be kicked out of the CMS interface.

Thanks.