I think giving up all food for something like Soylent is a bit silly, but at the same time, we do all make different sacrifices at times.
Everything I've ever seen about Soylent is along the lines of "now you can eat nothing but Soylent!", which, for how much of the world enjoys both cooking and eating, seems like a tough pitch, except for a very, very small niche that, I guess, doesn't actually get any visceral pleasure from food.
But as you point out, it would be interesting to see what the take would be as the occasional meal alternative. I wonder if it would be seen as things from GNC, etc. where there would be a small but interested market, but nothing larger.
They are still relatively young and will likely evolve into markets beyond the one they are targeting now.
Why is the message that it is build that you can do that a problem? (If I were to use it as a temporary replacement food I still would like to know that I don't have to take particular care to fix specific things it is missing. One less thing to worry about.)
That said, I think the customer base they are aiming for (and getting) right now is the right one for the moment (somewhat reliable demand from those that stick with it, willingness to tolerate hiccups, willingness to talk about it, ...)
Which, frankly, is going to turn a lot of people off.
One other remark--if you're skipping a meal on occasion, and supplementing it with some easy foodstuff, you don't really need to worry about it being nutritionally balanced. Virtually any western diet will get you enough nutrients on a regular basis that you don't need to supplement with vitamins or worry about the diseases caused by a lack of some nutrient, even if your diet is restricted for a while.
To me, Soylent is only interesting because they're wanting to replace every meal for normal adults living normal lives. Anything less is just better marketing of an Ensure clone.
While I try to eat relatively nutritionally balanced, I'm not perfect. I have a broken metabolism, and chronically bad knees. This makes exercise difficult to say the least, and whatever I eat will have an effect. What it did allow me to do was consistently stay under 2000-2400 calories a day, and actually drop a little bit of weight.
I'm planning on getting a gastric sleeve later in the year, which will make something like soylent ideal for me.
That said, I love to cook and usually do at least one longer prep meal a week (friday, saturday and/or sunday)... This past saturday I made mushroom broth, followed by an asian-style soba (buckwheat) noodle soup using some of said broth with thinly sliced sirloin and veggies.
So, I can really see both sides of this. On the one hand, I love food. I try 2-3 new to me restaurants a month, and like to take lunch away from my desk. On the other, sometimes just drinking something down is far easier than taking the time to prepare a meal. I've been making several gallons of stock/broth every other week for soup for most of the past year. It takes quite a bit of time and even only doing it a day or two a week, not doing it for half my meals appeals to me. And will often cost less.
Soylent works as a perfect meal substitute for when I'm lazy or short on time, and that means I have more time and money for when I want to have a nice meal.