Extra classes are a definite benefit to students because the lessons are tailored specifically to the needs of that student. This is also likely why students prefer them. They are not lost in a crowd and their needs are being paid attention to and more likely met. Extra classes often offer students help that allows them greater success during regular classes and boosts their confidence.
The problem is the lack of availabilty for this kind of extra help to everyone. Every student deserves an equal chance to get one on one or small group attention, but as mentioned above there is not the money or staffing to supply this to every student who wants or needs it. It does give students who have acces to these extra classes an unfair advantage. It is especially unfair when these classes are only available by payment, automatically excluding anyone in a lower socioeconomic group.