Something to consider, and disclaimer: I'm a systems administrator, who from time to time has to create such policies, let alone have to adhere to them myself.
I can understand where a company comes from with this; having a company issued laptop makes maintenance for such equipment not only uniform, but drastically cheaper, let alone the support for everything. Think about it like an airline; an airline can keep costs low (read: not pass on extra costs to the consumer) by having fleet commonality. Having the same model of aircraft for their entire fleet keeps maintenance costs and training low.
Wash/rinse/repeat here. If they have the same model of laptop or similar model of laptop from a given vendor, the cheaper things will be for the company, as well as being able to take that money and give the employees more benefits/wage increases, because they will have the extra money available.
Having that versus employee convenience shows you where the company is going and why they are doing it.
That said, this is a huge bonus for you. Think about it this way: If you are an application developer, systems engineer, graphics designer, etc., and you are coding applications for that company, by nature of the agreements you have with the company, your work is their intellectual property, and as such, you hold no personal claim to that property, despite the fact that you coded all of it yourself. That code is sitting on your personal computer, which they would have the right to take and make sure you do not have any versions of said code that you could take to another company, in violation of your employment agreement.
By having all of that code on a company laptop, you are providing all physical and legal separation of your personal work and company work. What is on your Mac STAYS ON YOUR MAC, and there would be nothing that the company could do to get that data or that code. Having personal work and professional (employee) work on the same piece of hardware is a major invitation to legal issues that an employee definitely does not want. It would be better to have the inconvenience of having more than one computer to haul around than to have those legal issues hanging over you, especially if it came to a very nasty separation of you from the company.
In all honesty, this is a blessing, rather than an inconvenience.
BL.