Thank you. It looks like the decision is made for me then. In order to use windows 7 on the 2015 rMBP I'll have to use a virtual machine in VMware fusion and enjoy the benefits of no HDD partition, use of all ram, and the ability to back up the virtual machine.
It seems there is no disadvantage to using virtual windows 7. Will mathcad run just as well in a virtual machine as using fusion to access it in bootcamp?
You'll see no difference, or marginally better performance. You were already running a virtual machine, you were simply using the more restrictive method of having one by having a bootcamp partition that is strictly accessed through VMWare.
That's what I'm trying to get through to you, maybe I'm not conveying my thoughts well, English isn't my first language.
The whole point of Bootcamp is to make a disk partition, so that you can install Windows onto it and boot INTO windows. Doing so would make your computer run solely on Windows when booted that way. Just like a normal Windows-based PC(Dell, Lenovo, you name it). Since only Windows is running, all the computer's resources(and thus, performance) are available to it. You can reboot the computer and go back to OS X once whatever it is you needed Windows for is done.
Same goes for OS X. If you are booted into Yosemite, then all the computer's resources (and performance) is available to Yosemite.
When you use VMWare fusion, the program uses the settings you chose to allocate resources to Windows, while OS X is still running. What that means is that if you have 16GB of RAM, and you let VMWare give 8GB to Windows (in your case, the bootcamp partition you are using as a virtual machine) then only 8GB is available to Windows, and only 8GB is left for OS X. Same goes for the number of processor cores you let it use.
If I understood you correctly, you used Bootcamp to install Windows on your machine, only to run it strictly through VMWare fusion afterwards. If that is indeed the case, then you took the long way around, and gave yourself less freedom with your virtual machine than if you'd gone 100% virtual from the getgo.
The bootcamp partition cannot be backed up through Time Machine if you use that. Should it ever become corrupted and you use no other means of backup, any data on that partition will be lost forever.
A truly virtual machine resides as a file on your hard disk. If you let VMWare do its thing, it will dynamically grow or shrink that file according to how much space your virtual machine requires.
The advantage is that since it is just another file, it can be easily backed up through time machine, and as a bonus, you don't have a partition with a set size taking up room in your hard drive, even if half of it is empty space.
I hope I explained things correctly.