Reiki classes are separated into different levels of training for a very good reason - to give the students body and biofield (aura, energyfield) time to process and integrate the fact that they are vibrating at a higher frequency then ever before because they now have the ability to channel the energy of the Divine that is Reiki.
We could liken it to a pie split into thirds - Level I opens one up to be able to channel one third of the Reiki energy we are capable of channeling in our dense human bodies. Level II opens you up to another 1/3 of the energy your system can channel (handle), and Level III (in my lineage we split the last level into two parts, IIIa "Reiki Master Practitioner", and Level IIIb "Reiki Master Teacher"), you are opened fully in the attunement to Level III to all the Reiki energy you are capable of channeling.
I find it hard to understand why some teachers feel it is acceptable to combine these classes, to me this is clearly about money and not about the best interests of the student.
Reiki Practitioners vibrate at a higher rate then those who are not attuned to Reiki. Our human bodies need time to adjust to this change in our biofield. I highly recommend that one experience everything that each level has to offer before jumping ahead to the next one. I instruct my students to sit in Level I for around 6 months (even a year if that feels more comfortable) and experience everything that this Level has to offer - being able to self-treat, Reiki your food, water, and whatever else you want to Reiki (medications, lotions, oils, crystals) and the ability to give Reiki to friends, family members, pets and plants and more, and to join in the Reiki Shares.
I would never tell my students it's time to move up a level either, in my opinion that is unethical. It is their decision and no one elses. In Level II you receive the Reiki symbols to work with, to focus the energy in a myriad of different ways and in most lineages this is where you are taught how to send Distant Reiki Healing. Again, I instruct my students to stay at this level and learn all they can about working with the symbols in their daily lives, in their self-treatments, and when they treat others. When and if they desire to move up to the Master Level, they know where to find me.
The last level of training, as mentioned above, opens you up to channel the full flow of Reiki energy you are able to channel and you get to call yourself a Reiki Master, whether you just want to do Reiki professionally (not teach just do sessions for others and charge for it) or go on to train to be a Reiki Master Teacher.
There are many other reasons why combining the levels does not serve the student and can be harmful, including the fact that each level of training and attunement brings on a 21 day detox or cleanse in all the major systems of the body. I went through a cleanse from hell when I was attuned to all 3 levels in three consecutive weekends because I foolishly did not do my homework and simply jumped into the first Reiki class I could find. I would not wish that on any one.