A 15" MBP is extremely difficult to use in coach class. There isn't enough room to put the screen at a comfortable angle, and there isn't enough room for your arms between you and the computer, so you will probably keep hitting the person next to you. This is one situation where a 13" would really come in handy.
Generally I don't use my laptop on planes, but now that they have internet, I might do so more often despite the discomfort.
The OP's concern is somewhat valid. Mechanical hard drives float the heads on a cushion of air, and that air cushion is the only thing that keeps the head from crashing into and destroying the platter. At some altitude there isn't enough air for the heads to float on, and trying to use one at that altitude could destroy the disk.
Most hard drives have an operating altitude rating (probably very conservative), and as far as I know, most (all?) of them are high enough to be used in planes. If brand X hard drives sometimes died during use on airplanes, I'm sure you would hear about it here with all the complaints people post. In fact, I'm sure there would be several threads per week about it. The fact that people use their laptops on planes all the time and I haven't heard of any failures because of it suggests that it isn't something to be worried about.