The argument against this is consistency. Without a limit set, you are only guarenteed up to your request's worth of cpu, but you will often be allowed to have more. This can create a false sense of security, as your application is working fine (even though it occasionally exceeds its request). Until one day, when a neighbor happens to get thirsty, and your application suddenly breaks. Limits front-load the brokenness so that it happens immediately instead of randomly.