It's not the much stress, I move several GB files around my hard drive constantly. We have computers at work that are used for backup that copy over 120GBs worth of files onto them on a weekly basis, and the hard drive is running fine after over a year. If your hard drive can't take the stress, then it is a bad hard drive.

Not to mention having a heavily fragmented hard drive means that basic read and write operations on it will require more work out of the hard drive since it needs to fetch the data chunks from all over the place rather than fewer continuous reads. And yeah this stuff really shouldn't be a problem. My 7+ year old laptop still runs fine and laptops tend to have crappier hard drives. And since it only had 10GBs of space I installed, uninstalled, and defragmented constantly since I couldn't have everything installed at once I just swapped things in and out as I needed them. Granted it hasn't seen nearly as much use in the last three years as when I initially got it but I still use it.