My understanding of a deferral is a bit different. It is more like getting an early offer of admission for the following year. Thus if you defer for a year you are not obligated to enroll in the program the following year. Well really you are never obligated to enroll in a program until you sign paperwork to that effect -- which, for many PhD programs, takes place when the student actually enrolls in the program. I also think that the most common reason for deferral is the OP's: that the student is just not fully committed to the PhD program she has been admitted to, and she hopes that the intervening year will clarify whether or not she should enroll. I think that a student should be pursuing other options during that year...assuming that the student and the program are on the same page about this.
As others have said, of course there is nothing like a right of deferral: if the application was not solidly strong then presumably the answer will be "No" or "Not without a good, specific reason" (e.g. health or visa issues). But I think that in many cases, an admissions committee can look at an application and say -- sure, we are confident that we would admit the student next year if they submitted the same application. By telling the student that now, we make their eventual enrollment in our program the path of least resistance.
Anyway, what's for sure is that in order to defer admission you need to have a serious conversation with the faculty of the program in order to make sure that you both understand each other and your commitments. In a comment on a previous answer I wrote that without mention to the contrary the understanding of deferral should be as in the first paragraph. Especially in light of Prof. Ismail's answer I now think that was a mistake. Sorry for giving bad advice in that regard.