You don't control them, they control you. In our den we've got an old blue (not bondi) iMac running OS 9. I've got iTunes 1.0 on there, and it does exactly what I need it to do: play CD's and MP3's with a snazzy visualizer. I'm sure Steve would barf dalmation chunks if he heard me say "iTunes 1.0 is just fine for me, thanks." And it isn't as though I don't love the latest version of iBloat, er, iTV/Movies/Podcasts/Tunes'n'Such-- it's just that in one case what we've got is just fine.
So with web apps you don't have the choice. It is made without your consent, your input and often without your knowledge until something breaks. Case in point: Gmail improvements. I had the first version of "hacks," including color labels, saved searches and labeling macros. Google finally caught a clue and added color labels, but to be fair, those were a very kludgey implementation. Saved searches? Well, that's nice, and once or twice I found them useful, but overall search is a temporal thing: you need it at the time and that's that.
But labeling macros, oh god, why must they be broken? Now I'm not sure if this is a Greasemonkey update thing or a Gmail "update" thing, but my productivity and GTD-esque workflow took a major nosedive when this broke today. I've studied user interface design, hell, I love a good UI (maybe why I love Apple so much-- when they do it right, and they more than often do). Have someone time you going to a checkbox, clicking it (or dozens), then navigating through a drop-down menu, selecting an item, THEN hitting a keyboard shortcut (y for archive). The alternative, or what the macro provides, is 95% keyboard: check the boxes you want, type 'l' then the first few letters, then 'y'... I'm serious when I say this is up to 50% faster. I practically live in Gmail, and this one thing is going to kill me. I actually developed wrist pain the last time Gmail broke the macro.
Which brings me back to my point: if a supposed benevolent dictator like Google, who "do no evil" but sure as hell don't listen to anyone other than the cabal of experts they make live in overpriced, insular Silicon Valley, if THEY change something for the worse, what can you do? Nothing. You have no control. The web app owner makes your decisions for you.
I think you should have at least the option of staying behind (and Gmail did have this option when they rolled out some big changes).
UPDATE: and just like that, the macros are working again-- go figure. Still, web apps suffer from this problem, and the problem of "how long do you support legacy code?" But even desktop apps suffer that issue (except that you can effectively freeze the state of an older machine once it is stable).
UPDATE 2: Chartier points out it is a gmail thing: as explained on their blog.
one man's journey into creating gibblybits