Modular mobile devices will be very nice because it will be able to create a phone that is perfectly suited to the individual, including only the parts of a phone that you want, and deciding the quality of those individual parts. Imagine first a teen who is tech-saavy. Maybe they want the faster processor and the high amounts of RAM, but could care less about a camera and a sensor on their phone. They could opt for a high end processor and a piece with a lot of built in RAM, without having to pay the cost of the included camera. Then you could look at an older person who is in the less tech-saavy category. Maybe they only really want a good camera to document family events and higher storage for that, but they aren't going to be running anything that needs high speeds. They would simply put in a part that has a good camera and another part with high storage, and opt for lower end parts to complete the phone. As you can see, modular phones are the ultimate solution to appealing to a wide-ranging market. This is also rumored to be incumbent in tablets, where the same principle applies.
And now, for where I think companies could take this promising technology in the future. To start off, I am thrilled that Google is working on this. They have the money and the minds to actually make something that will work. Plus, when Google enters a project, they are not always the first, and definitely not the last company to try and do that product, meaning more money, more thought, and more competition to build a viable modular mobile device. The one thing I haven't figured out, and what I'm actually looking most forward to is how they will make modular devices pretty. They can't work with the giant cases as you can with computers, and lets be honest, we all want a nice-looking phone, that's just half the status of owning one. Whatever way it unfolds, I don't see it in the near future, though it will be fascinating to watch the different projects unfold to delve into unexplored territory.