Well, there's truth to that statement, of course. However, I would submit that the vast ocean of concert band literature is written (perhaps 98% of it) for the educational market, which usually has an decent sized sea of clarinet players to shuttle off to the harmony clarinet chairs.
I'm an Air Force brat -- father was in the AF and we shuttled around all over the place -- this also means that I attended 5 different high schools.
All the high schools I attended had obscenely large bands. One was even so large that they had two "levels" -- something like "pops" and "symphonic", IIRC. Heck, my graduating class had 250 students.
Amusingly, in the school with the two bands, there was one bari sax player: me. My part was deemed important enough that I was in both of the bands. And jazz ensemble. And marching band.
So, when you've got around 30 or so clarinet players, you need to find something for them to do. In the case of the schools I'm referencing, they stuck 2-3 people on bass clarinet (one of who was me, for awhile), 2 people on alto clarinet and one brave soul on the Leblanc paperclip contrabass discovered in the storage closet (that was me, as well: I just thought the horn was kewl).
(My high school playing career was Bb clarinet, bass clarinet, bari sax, tenor sax, contrabass clarinet and then back to bari sax.)
As an aside, it was interesting that we didn't have anyone playing Eb soprano clarinet. We had two of 'em in storage. No one wanted to try, I guess -- which is the reason I started playing bari ("No one wants to play something that big! It's too heavy to carry home!"). No contralto clarinets around. Never saw a basset horn in our storage room, but there were scary brasses with four bells.
However, I attended large schools. Small school? Let's use my wife's school she teaches at, as an example. They have two trumpet players, a trombone player, flute, oboe, couple clarinet players couple alto sax players and a couple percussionists.
Challenging instrumentation to score for. I also think that the school owns ... a trumpet.
Anyhow, what Terry says is very correct: there is little to no call for "professional" alto clarinet players. Occasionally, someone will get a call to play contrabass or contralto clarinet -- I've seen concerts with these beasties -- but you're gonna get way more work with a bass clarinet or a Bb/A soprano.
So,
* I agree: most orchestration for alto clarinet is for "educational purposes only"; i.e. just "in case" you have people playing 'em -- and you'd have people playing 'em if you have a large band with a well-stocked storage closet (hey, the tenors I used were school-owned Mark VIs and a Martin Committee).
* I agree: one shouldn't aspire to play alto clarinet. If you wanna play and that's the only thing they have, go for it. But get chops on Bb clarinet or bass clarinet. Better, OWN a bass clarinet and be good at playing it.
* I agree: some very large military bands, particulary in France and England, may have been so large that they'd have alto clarinet players, just for kicks. I'd wonder if they'd be the first to get laid off when poor economic times hit
.