As an avid agilist, I maintain that the manager should do little more than putting the following three powers together.
* The techie.
* The funding.
* The customer.
These are the three roles to any "go"/"no-go" decision making on wether or not a particular technical endeavour is worthwhile.
The "funding" is simply someone who is able to decide cost against benefit. The financial backbone for a particular push within your portfolio is often quite obfuscated in a large organization. When you no longer produce bang for the buck, you stop.
The "customer" is simply the someone who can make a decision on a satsifactory solution to any given marketable feature. The customer represents the totality of the demands. In some cases, being an internal customer that is able to map external requirements to internal technical achievable backlog items is a true assett.
A good customer is hard to come by.
Together, the two can decide on where the efforts can give the most value.
The techie translates solution proposals into estimates.
The main discipline between these three is to identify the MMF's (minimum marketable features) that will individually add to the value of the product. It is sometimes hard for a customer to see that his problem is not an irreduceable all-or-nothing deal.
Again, consider the value of a good customer.
The customer is rarely an external person. When deciding on these issues, there should be as little vested interests (and politics) as possible. An external customer is likely to play up his requirements, is unlikely to be willing to vouch satisfaction at the drawing board and various scopes are subject to contract negotiations.
On small projects, one person can have all these hats.
On large projects, the "customer" can have a full time team of three people behind him doing nothing but mapping classification requirements to the proposed features.
Since the development departments are mostly techies, the two other roles are likely to suffer. Especially the customer part. We have many products where it is unclear who demanded what, and in what manner we fulfill certain requirements.
Every agilist should appreciate a customer who can think on his/her feet.