[lowndsy]

Six possible reasons off the top of my head:

If competitors can /are bidding on the brand name as well - having more bidders for your brand means you get more results.

If the brand name has a generic appeal as well, like "hoover", or "red letter day".

If the in-house ppc budget is exceeded by the possible daily expenditure, meaning that sometimes the in-house ads won't appear.

If a PPC partner is simply better at it than your in-house team, generating better conversion rates and ROI.

If the people in the closed group are also the top affiliates for generic terms as well.
A certain PPC person who I kidnapped in Barbados (you know who I mean) told me that the sad truth is that brand bidding is lucrative and easy, and a lot of affiliates/agencies only do generics to keep the merchant happy while they rake it in off brands - stop them branding and they'll probably stop the generics as well.

If a PPC partner is prepared to take you to search engines which your in-house team don't target - I'd say being promoted on MSN, ask, yahoo etc could be worth a compromise on Google.

A couple of the reasons depend on there being no agreement with Google to block the keyword, but surprisingly few merchants (imo) have done that anyway.