This most certainly is a Myth, and was for many years now. Apple does not have as many clones as there are in the PC world, of course Apple also doesn't have the quality control, installation problems, reliability problems that the PC world has as well (which come from cloning). But Apple does have clones -- and in fact Apple has more control over the clones, which means the ability to control and evolve the market.
Many want Mac clones because they think it drives down cost (due to competition) or because it helps the economies of scale.
What people do not realize is that you can get both those without cloning.
Remember the following two things:
So the assumption that Mac clones help either of those things (and cost) may or may not be true (there is usually a small grain of truth in there, that ignores the larger truths).
Does UNRESTRICTED cloning increase market share? There has been little proof of that in the Mac market -- and there is quite a bit of proof to the contrary.
The PC market got big because it was a standard -- because it had the IBM name (later Microsoft). The cost factors with clones certainly didn't hurt market share, but they may have hurt the market in many other ways (remember that PC's still have the same basic architecture they did 15+ years ago -- while the Mac has gone through 2 or 3 or more architectural changes). So there is little proof that cloning is what made the market as big as it is -- or that the same strategy could work in a smaller market. Sun has had an "open" architecture for a decade. Digital, Data General and others tried similar ploys -- and most have failed. So "Open" alone does NOT guarantee market share.
There are balances in all things. If the clones are only pirating Apple's sales, that reduces Apple's profits, and the amount they can spend on R&D and creating innovations that increase market share. Of course if there are more choices then there may be more market share growth. The trick is balancing the two against one-another. Apple tried to do it with unrestricted cloning, and failed -- now they are trying a little more control, and we will have to wait to see the results. Remember that Apple climbed from 0% market, to 20% with NO clones. Since "open" cloning they have dropped significantly.
Another argument is that Clones increase choice. This too may or may not be true. Many times lots of companies all go for the same market (or sub-market). This does NOT increase choice (in any significant way), and sometimes makes it unprofitable to produce a particular type of machine, and drives everyone out of the same sub-market. The results being less choice (short term at least). So unrestricted cloning has some disadvantages as well as advantages. Apple is controlling cloning, which means that companies that want to clone Macs must first find a was that they are expanding the market and increasing choice (or adding value in a way that Apple isn't) -- if they can do any of these things, then they will likely be licensed. That may mean more diverse choices as companies try to figure out new ways to diversify the market, and add value.
There are certainly trade-offs between restricted and unrestricted cloning -- but Macs certainly do have clones. To see a list of companies that either are producing clones (or have done so) then go to Clone Maker Index.