Disclaimer: the following post is referred only to the wine courses. I suspect that for Spirits and Sake the conclusions would be very similar, but since I do not have experience in teaching those I am not making assumptions.

I get asked this very often from prospective students: “What WSET level should I start attending?”
The question is legitimate because WSET allows you to skip Level 1 and Level 2 and to go directly to Level 3. Besides, most of the time Level 3 is what people really want: if you work in the industry holding Level 3 means that you are possibly getting some benefits in the salary and that you may start thinking of doing the Diploma, which is pretty huge in terms of the improvement it can bring in your wine career (it was for mine, at least). Add to this that the exam for Level 2 is a multiple choice test that has quite a high pass rate and the result is that many people, especially those who already hold different wine certifications like the Wine Expert or the Sommelier one by the Japan Sommelier Association, dismiss Level 2 as something that they do not need and go straight to Level 3.
Sadly, many of them meet a harsh reality on Level 3, they got completely overwhelmed by the program, they often fail their first exam and they have to resit it later, with less enthusiasm and lower motivation.

Now let’s leave Level 1 aside for a moment. Level 1 is a short introductory course which is great if you do not know anything about wine (really zero knowledge, not even self-education), if you want to try a wine course, but you are not sure about committing to a longer course or if you cover a position in the industry where wine is not you primary focus, but you would like to understand a bit about it (like if you are a cook in a restaurant for example). Level 2 also starts from zero, so even if you are new to this world you can still start from here: no prior knowledge is necessary, but the course is still 8 session long (+ the exam) and the fee starts to be relevant.

To give a quick answer to the question in the title: my advice as an educator is to always start from Level 2.
The point that I always stress to my Level 2 students is that yes, the exam for this level is comparatively easy, but the course itself is not. The course can and should also be seen as a preparation to Level 3. A preparation in terms of 1) calibrating the palate during tastings with the teacher, 2) understanding the approach of the WSET to study and of course 3) assimilating the concepts that are explained during the lessons.
Skipping Level 2 means first of all losing the chance to taste over 40 wines with an accredited educator, often the same person who is going to guide you through Level 3. Keep in mind that some of these wines will not be retasted on Level 3.
Second, starting straight from Level 3 implies that you already know what has been taught on Level 2, because the instructor is going to build from there. For example, on Level 2 we describe the winemaking process and we look at the entire flow from crushing to bottling. Level 3 on the other hand focuses more on examining specific stages, digging deeper on topics that were left aside on Level 2. You understand that to dig deeper students must have a good grasp of the bigger picture, like foundations must be in place if you want to raise a building.

During first session of the Level 3 course, I always like to show my students some of the review questions I ask during Level 2 lessons. For example “What are the main grape varieties of Eden Valley?” or “If I leave the grapes drying after harvest what elements are going to be concentrated inside the berries?” or “Where are the best Pinot Noirs found in California? Why are they found there?”. Usually I do not really ask these questions, but I clearly tell the audience that since they have Level 2 or they consider their knowledge to be on par with Level 2, they should comfortably be able to give me these information. The puzzled look on some faces is enough to tell me who are the students I should place particular care.
Starting with the second session, but particularly from number four onward, the WSET approach of interactive learning starts showing up clearly and it is at this moment that people coming from Level 2 find themselves more comfortable, because they are used to this style of asking-answering-explaining, asking-answering-explaining, asking-answering-explaining that they already experimented at a lighter degree during Level 2.

Is a student bound to fail if he or she skips Level 2? Not at all, I have had excellent students, brilliant people, that just started from Level 3 and then passed the exam at first try. However you should always be aware of what you are leaving out by making such a decision and what you need to do to compensate.
Since I have also seen Diploma students sitting back the Level 2 course to review the basics, I feel safe to state that Level 2 is more than worth the time it takes and that it should not be underestimated.