/64 prefix

more would be a waste of address space
my provider, tweakDSL, offers an ipv6 tunnel to their customers on request, which is set up manually for you by their admin, Friek. however, he will only give a /64. nothing more, not even a /60. Something similar may be done by other home ISP's. giving only a /64 and nothing more is not enough for comfort and unnecessarily restricts your customer.

"with a /64 you can number 2^64 of devices, which is enough"

Thinking with how many bits can number how many devices is a naive way of thinking, the ipv6 address is structured in many ways. Ipv6 address space is not measured in addresses, but in subnets. One subnet is a /64. A subnet is normally a single layer 2 lan without any routing or firewalling inside it.
Structure of an ipv6 address:

globally routable prefixcustomer prefixsubnet prefixinterface address
typically up to /32typically between /48 and /64/6464 bits

"why does a subnet need a /64?"

RFC 4291: "All Global Unicast addresses other than those that start with binary 000 have a 64-bit interface ID field." 64 bits for the host is specified by design, by convention, and in implementations. stateless autoconfiguration is more widely supported than stateful autoconfiguration (not supported by windows XP and 2003). being able to use autoconfiguration is convenient. being unable to use 64 bits for one subnet complicates the situation for the user and may cause problems, as in, it is uncomfortable.

"a home does not need more than one subnet"

You will need atleast 2 if you want both a wired lan, and wireless, and want control over how you set it up with routing, firewalling, public/private subnets, etc, or for experimentation/research.

"if you need those, you want too much for a home, and you need the more expensive business DSL"

Business connections give you, for a high price, access to resources which are really scarce or costly, such as: an ipv4 allocation, 1 on 1 contention, and uptime guarantees. a /60 allocation in ipv6 is not at all scarce like a /29 is in ipv4 so it is no comparison. it is generally agreed on that a home can have a small number of subnets.

"giving a /60 would be a waste. it is a scarce resource. the address space will be used up too fast."

all ISP's have a /32. /32 is the smallest prefix length which is globally routable without problems. With a /32, you can give the following number of prefixes to your end users:

prefix lengthnumber of allocations within a /32number of subnets in allocationcomment
/4865,52665,536officially recommended for large sites >100 subnets
/521,048,6754,096officially recommended for small sites <100 subnets
/5616,777,216256officially recommended for homes <10 subnets
/60268,435,45616given out by some ISP's for homes
/644,294,967,2961officially recommended only if one subnet is needed
conclusion: when giving out /60's, even with millions of customers, typical for large ISP's, you will never use more than, say, 2%, of your /32, that is not using any significant portion of the address space.

"i use 6to4rd to give ipv6 to my end users"

6to4rd requires 32 bits to point to the customer within the ISP's prefix. this is used by free.fr/proxad. however, they use a /28, to give /60 to each of their customers. i think 6to4rd is wasteful and is to be avoided.

sources

current recommendations for prefix lengths
RIPE policy for allocation to end sites
RFC4291 - ipv6 address design
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