This off the web:
The Hazard
Aluminum wiring, used in some homes from the mid 1960's to the early 1970's, is a potential fire hazard. According to the U.S. Consumer Product Safety Commission, fires and even deaths have been reported to have been caused by this hazard. Problems due to expansion can cause overheating at connections between the wire and devices (switches and outlets) or at splices. CPSC research shows that "homes wired with aluminum wire manufactured before 1972 are 55 times more likely to have one or more connections reach "Fire Hazard Conditions" than are homes wired with copper. "Post 1972" aluminum wire is also a concern. Introduction of the aluminum wire "alloys" in 1972 time frame did not solve most of the connection failure problems. Aluminum wiring is still permitted and used for certain applications, including residential service entrance wiring and single-purpose higher amperage circuits such as 240V air conditioning or electric range circuits.
Reducing Risk
As of the current date of this page only two remedies have been recommended by the CPSC: discontinued use of the aluminum circuit or, less costly, the addition of copper connecting "pigtail" wires between the aluminum wire and the wired device (receptacle, switch, or other device). The pigtail connection must be made using only a special connector and special crimping tool licensed by the AMP Corporation. Emergency temporary repairs necessary to keep an essential circuit in service might be possible following other procedures described by the CPSC. A special installation method is described in the article we posted on 1/31/96 below.
Here is the site:
http://inspect-ny.com/aluminum.htm
This may not convince your builder to change but it is enough ammunition to allow you to "be upset" with his choice.
With the price of new houses, I doubt this builder want's an unhappy customer, perhaps he will swap you for copper to appease you.
I did all copper except from the power transformer to the weather head. I scheduled copper to the weather head until the installer realized that the additional weight of the copper would pull my service box and meter off the house. Not wanting an additional creosote power pole in my back yard, I had to make that one compromise.
Where yours is run there is no reason except for the installer to save money. Copper is a lot more expensive.