I have a VPS with MediaTemple, they give me one IP to share with my own account and all my customers... Is it better to use the 1st IP provided by MediaTemple as my own and then purchase another IP for all my customers to share?
I would have to say yes to that. If for some reason one of your customers starts to get dos'd and you cannot handle it, you can have MT null the affected ip and your business will still have its presence. This is, of course, just one example of how it could be useful.
If MT null route the single IP, you won't be able to get in to fix your VPS (including Cpanel won't work if they null route the main IP even if you have other IPs to use). So yes, always put customers on other than the main IP.