I guess it depends on how you define "rare". I offer the following statistical analysis to suggest that, at least for the vast majority of posters on this board, the likelihood of having it is indeed very rare.
Consider that the number of Americans diagnosed with ALS in a given year is roughly 1 - 2 out of every 100,000. To be conservative, lets take the higher number and say our odds of being diagnosed with ALS in a given year are about 0.002% (2 out of 100,000). Perhaps the medical community would not consider this "rare", but it is certainly not as widespread as something like BFS which as many as 2 in 10 people may experience (20.0%).
In addition, most (but not all) of us also have to consider the following:
Percent of ALS cases that present with twitching first: Most estimates are between 0% and 6%. Let's assume the high end of 5%.
Percent of people diagnosed with ALS in their 20s and 30s: Hard to find reliable numbers. Definitely looks to be less than 10%, but lets go ahead and assume it is 10%
Percent of people with the inherited form of ALS: 10% (most of us can exclude this from consideration), so we only need to consider 90% of the ALS cases to begin with.
If you include just these numbers in the analysis, that number moves from 0.002% (2 of 100,000) to 0.000009% (9 out of 100,000,000). This is roughly where I got my 1 chance out of 10,000,000 number.
If you further consider that each passing month further lessens the likelihood that ALS will develop in us, we all ought to be feeling pretty good about the fact that we are highly, highly unlikely to have this disease. The statistics say we are all almost definitely suffering from BFS.