Ask your (potential) customers what would happen to their lives/business if you took your product away instantly.
Would it matter? How much of a bother would that cause? How many people would have to work how long at what tasks to compensate?
The more the problem, the more they need it.
The real question to ask is "Have you defined your product to be a must have?" I usually reserve "Must Have's" and "Nice to Have's" for features of my Must Have Product.
Defining your product requires you keep the customers needs in mind. That should be a given. Think of it from that angle and you should be fine. You can also check out this post on defining marketing requirements for some additional insights.
If it fills my needs: Must have...
If fills someone's needs: Nice to have...
I think there is a danger of thinking that "must have" is something you need to stay in business. It's not - a "must have" is something that has an important role in your business maximising it's potential. In other words, being all that it can be.
In my position, selling software to businesses, the "need to have" is when an app brings real value to a business. In other words, this app helps me make more money. Afterall, an investment in software is something that has to be judged by it's return - the money it costs has to be balanced against the money it makes. Often this is in an indirect manner such as increasing the efficiency of staff or improving communication but still plays a part in my business making more money with it than without it. In opposition, "nice to have" is window dressing.
Tough to literally think of most products outside those that provide basic needs (food, clean water, shelter, safety) as must have, so I'll define must have as, my life would be tougher without it.
I'm guessing your users will score on some scale from useless to must have. The key will be getting enough people in the must have. They will be the early adopters and provide feedback both good and bad (Bad is a good thing if you can utilize the advice and improve your app.). Others will try your app, probably not buy it and give you no indication what the problem is.
If you have a web-site to provide a service, you have a nice but drastic option: pull the plug and see if there are any angry emails or calls.
Seriously, the only way to know is when the users tell you. They can tell you in many ways. The best way is when they tell you with their money. If your product is currently free, what do you hear from the users? We just rolled out a new product, it's free for now, and here is a bunch of comments we are receiving every day:
and so on. We are pretty sure that it is a "must have", at least for some people out there.
A recent book I read is called Trade-off by Kevin Maney. I'm always VERY skeptical of any author writing a "business book", when the author has never run a business himself. This book isn't gonna make you rich as soon as you read the last page and execute some blueprint, but it does offer some food for thought relevant to this discussion.
This book postulates that great businesses make a choice, they either focus on making something ultra convenient or they focus on making something "high fidelity".
High fidelity is a bit hard to define, but you can tell what it is from examples. A concert with all the noise, fans, beer, dancing is high fidelity. Buying an MP3 that's convenient. Going to Harvard is high fidelity. Taking classes online at the University of Phoenix is convenient. An iPhone is high fidelity; the free phone you can get with any cell phone plan is convenient.
I think it's a great thing to keep in mind when you are creating your product. Whether or not its a nice to have or a need might not matter. If you have a nice to have product on your hands, you might still have something pretty powerful if that product is on the extreme of being high fidelity or extremely convenient.