I agree that when it's needed, it works.
I do also believe that a LOT of kids who are getting speech therapy don't really need it. Parents will do it because they're worried and because it certainly won't harm the kids, but like mamadelbosque says, there are SO many kids out there who blossom on their own just fine, just on a later schedule than what is bandied about as 'normal'.
Personally, I think that it's not the kids that are "late", it's that the idea of "normal" is mis-represented. The guidelines for how many words at what age, etc.
There ARE cases where there is a legitimate speech development issue that needs assistance, but if you think about it, would it make sense (say, evolutionarily speaking), that such a large percentage of human children would need professional help in learning how to talk?
There needs to be assessment done, for sure, to identify the legitimate issues, but it needs to be MUCH more than just "how many words by what age". My impression has been that there's a lot of kids in speech therapy at 20, 18, 15 months old for no other reason other than their parents have friends with a kid the same age who talks a lot more than their kid, or because some doctor looked at some chart and said "if she doesn't have 12 words by next month we're getting her into speech therapy".
And there are ladies like dutchgal who are therapists who are sincerely trying to help the kids who need it.
I'm assuming that they also tell parents who are just paranoid or misled, that their child is fine and doesn't need any therapy. I'm also sure that there are therapists who will take on any child whose parents ask for therapy, whether or not they need it -- because 1) it doesn't hurt the child and 2) the therapist needs the work, after all. So this could, perhaps, artificially inflate the apparent number of kids who 'need' it... because the child will develop their words, the parents will be satisfied, and it will go on the records as another child 'helped' by speech therapy.
Honestly, I think this is unfair to the kids who actually DO need and benefit from therapy.
Anyway, I don't have the research and numbers to back this up, this is all just my impression, what I've seen from anecdotal stories... "Oh, my child started therapy at 20mo with only 6 words and by 24mo she was speaking in full sentences!" Yah, well, that's a pretty common thing to happen without therapy too. So I'd actually be very curious to see what the evidence-based research shows, as to the actual difference between kids who get speech therapy and those who don't, among a normal population. How many of the kids in speech therapy these days honestly need it, and how many are there from misconceptions of what is 'normal'?