EBITDA is a good un-levered metric because it approximates cash flow. For a ten-year old company going public, actual cash flow and net income are more important. If Dropbox are trying to price at a premium to Box and don't even have profits, they're going to look like Snap and Twitter more than anything respectable.
Kinda. It approximates cash flows for some types of companies. For others it doesn’t. Hard to generalize here. Ultimately the best measure of cashflow is cashflow. It’s a good normalized measure of a stable mature business, but it can overstate or understate the health of depending on the revenue and billings model. So, basically, YMMV
Bit of a trick: build your own cloud and turn opex into depreciation on capital investment = become "profitable"