Since the introduction of the Waves NX, a virtual monitoring plug-in that simulates the ideal acoustics of a high-end mixing room inside your headphone, I have been getting more and more questions about whether is it really necessary to get a pair of studio monitor speakers for mixing anymore? The short answer is yes, you should while the long answer would be that you should not entire depend on your studio monitors to get a comprehensive view of your mix. This would also answer the question above, the Waves NX plug-in is very revolutionary in its field, however, the same practice still applies nonetheless, you should not just monitor on one medium alone.
Studio Monitors are loudspeakers specifically designed and engineered for audio production applications such as recording, mixing and broadcasting. As compared to a consumer level loudspeaker such as the widely popular, Creative Inspire T6300, a professional level studio monitor will have a flat frequency response and a wider dynamic range. Simply put, a studio monitor has a more relative linear reproduction in frequency responses (e.g. no bass boost), thus providing a more accurate image of what is being recorded or edited. This is essential as it will reveal the flaws or problems in a recording more effectively than a consumer level speaker. Secondly, if you ever tried mixing through a Hi-fi speaker, you probably would know that is a very bad idea (personal experience). Studio monitors are made in a more physically robust manner than Hi-fi speakers as they often only designed to reproduce compressed commercial recordings (after mastering). On the other hand, studio monitors are designed to have a wide dynamic range to cope with the high variations in sound levels often found music, example, a sudden drum introduction.
Getting a pair of studio monitor speakers does not entirely solve the equation as mentioned above. In many cases, most engineers will record and mix on the highest fidelity medium available at the beginning phase of the project. This is coupled with the highest possible recording format (e.g. 48KHz , 24 bits or 96KHz, 32 bits) that the production team has decided so as to ensure the best possible recording results. However, studio monitors are just 50% of the whole picture in this case. In a real world application, the room the monitor speakers are placed in often play a huge contributing role in affecting the overall sound characteristic. In general, know your room well, the smaller the room, the more difficult it is to monitor the low frequencies accurately. Therefore, I would often suggest not to splurge on monitor speakers alone, but if you finances allow, invest in some room acoustic treatments as that will provide a more comprehensive system. Any monitor speakers in a fairly non-reflective room will provide merits that will easily justify the purchase.
The journey has not ended once your project has been mixed. You need to understand that the general public does not own a pair of studio monitor speakers, in fact, you mix often do not get represent accurately in their listening medium (e.g. iPod earbuds, laptop speakers). Hence, many engineers would also sample their mix on cheap speakers to get a picture of what their audience would be listening to. In my case, I often have a cheap Logitech active desktop speakers connected to my DAW so that I can easily do an A/B comparison on how the mix would sound on different speakers. This test should not cost you much as the main idea is to obtain a widely available average performance speaker that would most likely be used to listen to your music (e.g. your phone, tv or any existing playback devices).
In conclusion, studio monitors are still very relevant in today's music production work, it gives you a highly accurate picture of how your mix would sound like. Nevertheless, always have the habit of sampling your mix on the different platforms, as that will provide you with a more comprehensive evaluation.