There are certainly benefits to sticking with one style of yoga—digging a single deep hole rather than several shallower ones. The practice even within a single system is not necessarily uniform. In Ashtanga, the set sequence of postures can function like the “control” in a scientific experiment, highlighting changes in the practitioner. Iyengar Yoga has within its defined tradition both very vigorous and very gentle sequences, which are selected to meet a particular student’s needs.
There is also validity, however, in being a yogic gourmand who delights in many tastes, styles, and approaches.
If one day you want to sweat and strengthen your way through a Power class and the next day you feel more like being guided by your Yin teacher into a long, deep Pigeon Pose, you can choose appropriately. Varying your practice can be a way to get in touch with your inner guidance, foster balance in your body, and bring a quality of richness and spaciousness to your yoga.
A school that has a range of offerings can provide several doorways through which practitioners can enter into a yoga practice, as well as various places to make transitions along their path.
You may find that, after several years of experimentation, one method really captures you. You may decide to give yourself to it completely. Or, you may dedicate yourself to a particular system, but as your circumstances (such as age, health, or energy level) change, you may find that you are drawn to other styles.
No one has the authority to decide the right way for anyone else. Yoga is fundamentally a deeply personal practice; no one system or approach is better or worse than any other. If you are growing through your yoga and love doing it, then it’s working.