Most of us pilots in my sphere of influence do not believe there was any climbing and certainly no rapid descent. It would not have served any purpose unless there was a struggle involved. Remember the transponder is off and Malaysian military radar can not accurately know for certain altitude from the distance all this was suppose to occur.
I agree there is no obvious need for climbing to 45,000 feet, or for a rapid descent (and no it couldn't have been 40,000 ft/min as reported), though I can think of a few possible reasons. Distracting the passengers and flight attendants, making them think maneuvering is in progress in the front so take your seats and strap in, avoiding a sighting from another aircraft, confounding military radar operators trying to plot a course, among others. But certainly it was not needed to kill the flight attendants and passengers. Manual pressurization could have taken care of that, much more quickly and effortlessly.
We have to accept as fact what the Malaysians (confirmed by the Thai's) said about the flight path. True, the altitudes aren't necessarily accurate but military air defense radars are designed to detect altitudes too -- unlike civilian (primary) radars. Think about it -- if you are about to direct an intercept you need an altitude. The altitude (and to some extent the speed) could tell you a lot about what kind of threat it is, and what its intentions are. You direct the interceptor to the target, and the interceptor locks on with its own radar when it gets close. But you need more information on the target than 'surface to 45,000', of course.
I don't know exactly what kink of AD radar the Malaysians have, but someone posted a link to an article about ours (entire US coastline), and it definitely can discriminate altitudes.
And the humorous or sad part depending on ones point of view, Malaysia claims of altitude over land has been a moving target. Reports of 5,000 to whatever. One thing i am very certain of, it was never less than 13,000 feet. And what i do know, most likely higher. And as far as routing, Thailand ATC is only acknowledging the airplane's path south near the border of Malaysia and Thailand. Saying it never ventured into Thai airspace. They are quiet on any turns toward Phuket and the Andaman Sea. So even the announced flight path is brought into question.
The handshakes from the satellite gave it two options, the northern route would have taken it over northern Thailand. We know for certain that did not occur. It never entered Thai airspace.