It is a good idea to replace soda bottles with some frequency, even though they can take a lot of abuse before they bust. Since they cost roughly nothing to replace, this is not a big problem.

The following "factoid" came to me as a bit of unverified lore, but it makes perfect sense, so I believe it. Bottlers have no love for wasting money. Therefore they make their bottles only strong enough to do the job, no more.

The weakest part of these PET bottles is the cap and its seal. However, because carbonated beverages are under greater pressure, the cap and its seal are stronger on soda bottles than on, for example, disposable water bottles, and will leak less readily.

There. Believe it or don't. I can't back it up.