I think there are two important points that are being missed here:
1) The problem with crunches isn't that they are inefficient, or don't work (on your rectus abdominus and obliques, primarily), or can't, along with lowering your body fat, give you visible abs. The problem with crunches is that they put -- again, according to Dr. Stuart McGill -- unbelievable pressure on your discs. His studies have lead him to believe that it's not a question of *if* you will damage or even rupture a disc doing crunches; it's simply a question of *when*. The reason to advise against doing crunches is that they will likely lead to injury. (I encourage you to google Dr. McGill to read more about his research, positions, etc.)
2) The *core* and the *abs* are simply not interchangeable ideas, and while crunches do indeed work your abs, they do not work a good portion of your core. As Schuler and Cosgrove discuss in NROL for Abs, the core includes a number of muscles in your back, around your torso, leading up your spine, and down into your pelvis. The purpose of this musculature is to lend support and stability to the spine, NOT to flex you forward. To work the core is to work that entire musculature towards the purpose of stability, and this is why planks and related exercises are good for developing abs AND the remainder of your core as well. This is also why the above poster who mentioned heavy squatting is correct; doing loaded work like that where you are struggling to keep your spine neutral and stable is a very good core workout.
So, yes, I would say that with diet and crunches, you can build visible and even strong abdominal muscles. By avoiding crunches and doing stability work for your entire core, you can build not only visible abdominal muscles, but a stable mid-section that will better resist injury. And, regardless, you need to be lean to see those abs.