I think maybe a lot of people try to fill or fix cracks between boards that vary significantly during the year. These kinds of problems are really not fixable. They come from either incorrect installation or just because the wood was not t&g or acclimated etc. This periodic swelling can be really bad if fixed in the manner I described - especially if there isn't the usual 1/2" space left all around between the floor and its surrounding walls for safe, normal expansion.
The filling techniques I cited work just fine in a situation where the floor was installed correctly and the wood was aclimated etc. I have also used it in very old homes where the floors were stable. I have seen this done by all of my floor subs in Syracuse for new floors, and in the Boston area for new and ancient floors. Again, you are right, it will not work if there is a shrink/swell crack problem.
If you don't use any filler your floors must look pretty crappy. What do you do, leave cracks and holes and imperfections just staring at you. This isn't very professional. I still stand by my filling method for floors that don't have any problems (I should have added this proviso originally). I have overseen it done on about 30 different projects and I have never received any complaints or problems. The subs I used have done it to over 1000 floors and have not had problems. Stable floors with expansion room around the edges can be filled, no problem. I'm not talking about HUGE gaping holes. That is a sign of a problem floor.