I'm an interdisciplinary PhD student and I've heard conflicting reports on your publication record in your early career. Some members of my host department say that you should stay focused while others outside the department say it can help. Given that my dissertation is by publication and I've got one core field and two secondary, what would a hiring committee expect my CV to look like? Would publications in tertiary field (as primary author) be looked upon negatively?
If you have 5 publications in 5 different areas, I would be concerned that you might become the eternal prodigy or that you are not willing to perform some long term research. However, if you have a clear focus and can document that you are working continuously on one project, then activities outside your core area indicate that you are able to work together with very different people. So you won't be hired because you have many publications, if there is no area where you are really convincing. But among applicants, which are similar in the area the position is aiming at, other activities become a significant advantage.
TL;DR: If you just start your PhD, it's better to stay focus on one topic.
You can imagine yourself as an employer (whether in academia or in industry), looking for a candidate to work on topic X. There are two CVs, one has 5 papers on X, and the other also with 5 papers but on X, Y, Z... Which one do your prefer?
After PhD, you are supposed to be an expert, having deep knowledge on a (very narrow) topic. You need to make sure that you have enough publications to prove this. If you wrote your thesis as a monograph, you would need to tell a story, and only the papers relating to that story should be included in the thesis.
I have no idea how a compilation thesis looks like, but I imagine you still need to write an introduction, which summarizes all the papers, and how they are related together.
Moreover, focusing on one field will make you more productive. If you can publish 5 papers in 5 different field, then you might be able to publish 10 papers in 1 field.
There are a couple aspects of this to consider:
Some members of my host department...
They are your host department, but you're an interdisciplinary PhD student. There is not promise that your host department now will be the kind of department you end up sending job applications to. I can tell you, in addition to my "primary" field, I applied heavily to positions in other areas, and when it came down to it, it was one of these applications that landed me my current position. Papers that are slightly off your main topic area help signal that you can do work outside that area. It gives people in those fields something to grip onto and evaluate while they consider you, etc.
...you should stay focused...
The key here is not to let side-projects distract you so much that they start obstructing your actual progress. They are, inherently, side projects. You should have some way to tie them into your main research agenda (even in different fields, what you do should have a coherent theme) and participating them should be holding you back. But a paper or two in adjacent fields likely won't hurt your chances overall, and in some cases may help.
Everything else equal, publications outside your primary area are unlikely to hurt. If you have a strong record — e.g., good publications, evidence of teaching excellence, a clear scholarly identity, and a great letters — publications outside of your area aren't going to disqualify you. Some departments are more method focused and interdisciplinary in ways that might even cause "side" publications to be helpful to some degree.
Generally, publications outside your field will help much less than articles in your field. Hiring and promotion committees will have a much easier time evaluating your work in your area. They know the prestige of different journals, for example. Additionally, publications within an area help build a consistent scholarly identity which is helpful when you're selling yourself to employers and you're fighting to build a name for yourself. Fantastic letters often say things like, "X is among the best young scholars working in the area of Y anywhere." That's hard to write if your scholarship is all over the map. You have limited time and energy to work on publications. Use it well!
In the absence of a strong scholarly identity and clear evidence of excellence, "side publications" might be seen interpreted as evidence of a lack of a focus. Hiring and promotion decisions are not made based on a single criteria. Although publications from an outside area are not likely to hurt on their own, they might be taken seen as additional evidence of — or an explanation for — weakness in your application. If you haven't published very much in your primary area, folks are going to think your side publications might be why. If your commitment to your discipline isn't clear from the rest of your record, publications outside your discipline won't help.