First we must understand how force is transmitted through the reel from pressure on the line. Line is putting pressure on the spool from the fish hooked to the other end. This causes the spool shaft to push against the bearings seated in each sideplate. It also pushes the pinion gear into the main drive gear a little harder. This puts force on the main crank shaft running through the roller bearing which is also seated in a sideplate. So both side plates see pressure, as do the gears and bearings and crank shaft. The crank shaft is also seated in the frame, so the frame sees pressure from this as well, and also at the point where the reel foot seats into the reel seat on the rod. Still further, the frame sees pressure from the points of contact between it and the sideplates also. Pressure is dispersed pretty widely in a reel when under load. The harder the fish, or you, pull, the greater the forces are.
Now, remember when we were kids and we'd find a stick on the ground and break it up into progressively smaller pieces, and the smaller it got, the harder it was to break again? Sooner or later you'd reach a point where equal pressure each time is no longer enough to break it, and more is now needed. Same with side plates, they're pretty small, relatively speaking. So how much pressure does it take to flex one? I'd say quite a lot, especially if the reel's frame is aluminum, but much less if the frame is graphite since it will also flex with less pressure as well. I believe that's where most gear misalignment comes from. Reels under heavy load and not rigid enough to take it.
This is also why all graphite reels will sometimes last a long time for some guys who don't catch very large fish very often, or guys who are light on their equipment in terms of use and abuse, and even guys who use better fish landing techniques. It's why one guy will tear up a reel and say it's junk where another will use it indefinitely with no problems (aluminum frame or not). It all breaks down to various pressures applied over a reel's lifetime in how long it will last and whether it flexes, or even fails, or not. For most people, an aluminum frame with graphite sideplates is enough for about any fish they'll catch.
I think the lion's share of the market is basically established by price, and various priced reels give you varying degrees of quality. People usually choose a reel based on price, but I think it should be chosen based on how hard you fish, how often, the type of fish you persue, and how abusive you are to your gear. This should all play into decisions on whether to buy all graphite or all aluminum, or somewhere in between. I think for the most part, an aluminum frame with graphite sides is strong enough to hold up for most people. That's about as simple as I can explain it.