Do I need a seperate domain name?

I currently have two domain names. The first is my website and the second is used as a re-seller account that was shared with a few friends. Although I am now on my own, I have decided to retain the second domain name for the following reasons:

1) In my environment, all ports except 80 and 443 are blocked. This means I will not be able to access:

Port 2082 -> Cpanel default port
Port 2083 -> Cpanel over SSL
Port 2086 -> Cpanel Webhost Manager (default)
Port 2087 -> Cpanel Webhost Manager (with https)
Port 2095 -> Cpanel Webmail
Port 2096 -> Cpanel secure webmail over SSL

WHM has now included a feature to access the above using port 80.

WHM includes a feature that lets your customers access the following from behind a firewall, using port 80:

•cpanel.example.com — The customer's cPanel account.
•whm.example.com — The customer's WHM account.
•webdisk.example.com — The customer's Web Disk account.
•webmail.example.com — The customer's webmail account.
(In these examples, example.com stands for the customer's domain.)

You can enable this feature by selecting On for the follwing Tweak Settings options in WHM:

•Proxy subdomains
•Proxy subdomain creation
You can learn more about these options by viewing our Tweak Settings documentation.

Or, if you prefer, you can use /scripts/proxydomains to reconfigure the DNS entries manually.

Source: http://docs.cpanel.net/twiki/bin/view/AllDocumentation/AllFAQ/WHMsFAQ#Is there a way to access webmail

Hence, a domain name is essential.

2) I cannot use my acutal website domain name because my current site is already live. If I were to use that to configure my WHM and cPanel, there might be a lengthy downtime. Changing the name servers alone may already take up to 48hrs! I also intend to update my wordpress and plugins and this will take up more time and increase the chance of errors.

3) An additional domain name is useful for testing purposes. Although I can create subdomains later on, I would prefer to create them under a seperate domain. By testing on a seperate domain, there will be no chnace of corrupting any files in my live site.
There are some considerations when getting a separate domain name:

1) Cost: pretty obvious but fortunately the cost of a domain name is relatively inexpensive.

2) Privacy: ICANN requires that every domain registrar maintain a publicly viewable "WHOIS" database that displays personal contact information (including home addresses and phone numbers) for every domain registered. Most domain registrars now offer WHOIS privacy at an affordable rate.

3) Vulnerability: An additional domain name means another avenue for an attack. Therefore it is important to find a realiable domain registrar. Some even provide additional security measures but at a cost.

Having another domain name is useful for people like me who are not very knowledgable in web hosting technoglogy, require as little downtime as possible or as an added insurance (testing). If you are on VPS, I believe you take your site seriously and having a seperate domain name will come in very handy.