No you wouldn't, you think you would, but you wouldn't.
1. If you lose your phone, then all your work is gone.
False. Ubuntu has cloud storage built into it if you choose to use it.
2. If your program is going to run an anything but your phone you will need to move it off to another system anyways.
LOLWUT? You can compile for target architectures that are different from your own. This has been built into compilers for a very long time now.
Unless you are aware of this and meant that if the target architecture is anything other than your phone, you will have to move it off anyway. Still -- why would that matter? By the way, why would someone need to do this anyway? Canonical is shooting for a complete solution -- i.e. your phone IS your desktop when you need it to be.
3. Oddly enough you will not be happy with mainframe only features you are going to use the extra features your phone has and slow it right back to mainframe speed. At least the mainframe is designed for many people using the system anyways.
The latest phone architectures have quite a bit of computing power built into them. With smaller process manufacturing on the horizon, I'd say we'll see that power go up quite a bit soon.
4. Why the hell is your university still teaching software development on a mainframe, That was so out of date 20 years ago!
Not exactly. Mainframes still exist today. But just because programming work is being done on a mainframe doesn't mean much (see my reply to #2). But even still -- especially with intro classes, this is a very good thing because it puts all students on the same platform with the same guaranteed experience when they go to compile.