You are talking about a U.S. passport of course; you may note that some of the posters are members of non-US programs, and hence likely to have other-than-US citizenship :-) Many countries don't allow for extra visa pages. Some give you additional passports (in some cases the passports are fused together).

The U.S. does have a limit to the number of additional 24 visa page inserts allowed. One used to be able to get these extra pages at every US consulate around the world (or in the US, the center in Philadelphia/Delaware). http://travel.state.gov/passport/fri/add/add_850.html

Now, most consulates will only give you a first or second set of extra visa extension pages; I run out of these rapidly, and once I got to 5 sets of extra 24 pages inside my 48-page passport, I was generally refused and told that I should do this in the U.S. While nominally these pages are free, expedited service costs $60 plus FedEx both ways. Since my passport is seldom in any one country longer than 3 days, this presents a hassle.

Once I got to my 9th additional set of visa pages, even the passport agency balked. True, he passport had started bulging obscenely and coming apart as well, presenting potential problems with an overzealous immigration inspector in some random country. So, unfortunately, even though I still had a good 4 years left in it, I had to renew. I still feel like a virgin. The biggest hassle is in transfering multi-year visas from the old, cancelled but still-unexpired passport to the new one.

Also, they have stopped issuing the 48-page passports, so what they now do to accommodate frequent travelers is issue a 24 page passport with your first set of extra 24 pages. From http://travel.state.gov/passport/fri/add/add_850.html:

Argh.

I did discover a while back that one can request a second, valid, passport. You need to explain why you need that passport, but if you have an existing passport that's bulging with visas, the explanation is fairly obvious. With a peripatetic travel pattern, it becomes quite a challenge to obtain visas. If you're going to most normal parts of the world where you don't need visas, it's not an issue. But if you're visiting various African failed states and kleptocracies, Arab countries and rogue states, even with good visa expediting services (I swear by TraVisa), it sometimes takes a week or even longer to get these visas. Hence a second passport is required. The U.S. does issue these passports (2 year validity) for cases where prolonged delays are anticipated for a required visa. Documentation is necessary to prove travel requirements to such countries where visas are slow to obtain.

A second passport is also mandatory in the case of certain "tit for tat" countries that due to whatever confrontation is going on between them, country A refuses you entry if you've been to country B (i.e. a stamp from country B in your passport). Our passport agency does understand this. For example, you can't enter Syria or Lebanon if your passport shows an Israeli passport stamp and the US will issue a passport for this purpose, limited to two years. Second passports are, however, extendable if required for additional 2-year periods. You need to show documentation of upcoming travel to two countries embroiled in conflict. Another example is Cyprus; if your passport is stamped in Northern Cyprus (occupied by Turkey), you may be denied entry into Cyprus. A normal Turkish stamp is no problem.

Hint: Countries whose passport stamps may cause problems elsewhere will generally give you the option of either not getting your passport stamped, or getting a loose sheet of paper (in some cases a plain sheet, in other cases, a special form) stamped so as not to inconvenience you. This is an option at Ercan airport in Northern Cyprus, and it is an option in Israel. It is also an option in countries who allow Americans, but where the U.S. Government forbids or restricts travel (example, Cuba).