The appeal of a combined bachelor's/master's program is that it is efficient. Bachelor's degrees have some slots that are filled with electives that don't really contribute to the student's knowledge of his discipline of choice. A combined bachelor's/master's program lets a student fill these slots with a few graduate-level courses, even though he's still an undergrad. Since he's going to go onto a master's degree anyway, this makes the master's program shorter.
Bachelor's/master's programs are not generally applied for right out of high school. Rather, they are applied for in the student's third or fourth year of study. This is because the bachelor's/master's program doesn't change your coursework until these years. What's more, most students cannot confidently say that they are passionate enough about their discipline at 18 to want a master's degree in it.
A student in a combined bachelor's/master's program needs to take graduate-level courses before he is finished with his undergraduate work. This means he needs to discuss ideas with and compete with students who already have their bachelor's degrees while an undergrad. So, students in bachelor's/master's programs need to have substantially above-average academic aptitude in order to succeed.
Students with clear academic and career goals reap substantial benefits from these programs, particularly those who want to pursue a Ph.D. and go on to careers in academia. Since a Ph.D. takes so long, being a year ahead can make it substantially less harrowing.