Using Similar Hardware Model of Computers in an Enterprise Environment

Our company has been using the IBM Lenovo R and Z models for our laptops. Now, to cut cost, management is thinking about buying different brands and different models of laptop depending on specifications.

I am completely against this because I believe IT can work more efficiently using similar hardware through out the Enterprise and keep uniformity.

I need to justify my case. What do you think are the advantages and disadvantages of working with similar hardware in an Enterprise environment.

Thanks.