No particular reason to change increment to anything other than one. It was the only term that wasn't variable. 

An increment of one seems like a good constant, but I did think of a couple of use cases where it makes sense to have an increment other than one. For example, when configuring a number of small subnets (netmask 255.255.240) one might wish to set $ip_incr=16 to setup all the gateways. Or, use an increment of 2 where every  even address is a test-server for the previous odd address. 

However, I can't myself imagine changing it.