I get why they do it, but actually causing issues just to cycle IPs seems like a lot. I've had cases where I have the same IP assigned to me for quite a long time.
They already did this on dialup and ISDN, back when routers didn't exist. When DSL first started appearing they just kept doing it. The idea was to not keep unused connections/IPs up, since you didn't have a router you would need to reconnect manually.
Also, it made it harder to run servers until we had services like DynDNS.
On newer connections it isn't common anymore, especially if you have a voice service.
