In any field, as long as the terms we use are well defined and we use them in a way that maps onto reality well then they are useful and we should use them.
There is certainly a wide range of things to describe in sexuality and it seems that there are few total absolutes. It may be the case that few people, if any, are 100% straight or 100% gay. But as long as the notions of gay and straight are not absolute ones they serve us quite well. Most people have either the vast majority of their sexual attraction derived from the opposite sex or from the same sex, so the terms gay and straight describe them quite well. If someone uses words like gay and straight as absolute terms meaning someome who always has all of their sexual attraction derived from either the same sex or the opposite sex and could not possibly ever be attracted in any other way, then the terms do not describe as many people as well.