There is an argument that Macs do not have enough OS's. This is an argument by the PC types that are trying to put down the Mac. The flaw in their reasoning is the question "How many OS's do you need?".
So what are they really saying? That the needs of 1% of the users make the Mac bad for the other 99%? That doesn't make sense.
There are certainly a few that want or need alternative OS's, or to run Software that is only available for an alternative OS. That is a valid point, but (by far) not the norm.
For the people that want to run alternative OS's, they want do do so for one of the following reasons: Compatibility, compatibility or for compatibility. Sometimes they want the compatibility to run a 3rd party app (like a game), or sometimes for their older legacy app, or for some specialty application -- but it is almost always for app compatibility.
For those few (1% of the 1%) that want to run an alternative OS, for some reason other than compatibility -- it is usually because they are programmers or hacks, that want to toy around in another OS. The Mac often has plenty of choices there anyway -- but 99% of this group really only wants to run a flavor (any flavor) of Unix. Well the Mac has many flavors of Unix available -- so they are covered here as well.
More on Compatibility
The Macs are often the most compatible machines on the planet. They can run PC software just fine (through software emulation, Hardware cards, remotely, or native) -- but the PC's can't really run most Mac software. (Yes there is one Mac-Emulator for the PC's -- but it doesn't work very well, nor does it run many Mac Apps, nor can it cure PC's inherent weaknesses in other areas). So for the most part, compatibility is one way -- towards the Mac.
Most PC users that buy Macs THINK that they will need PC compatibility -- yet when they get the Macs (say one of the DOS compatible Macs, or buy SoftPC or an emulator) they quickly learn that they spend 95% of their time on the Mac side. There are plenty of Mac applications -- and most of them are "best of breed". So the need for PC software for general usage is almost nonexistent. It is only in specialty apps that it matters. For most of the specialty Apps, then software emulation offers plenty acceptable performance (so for $79-99 they have everything they wanted out of a PC, and far more).
It is rare indeed that people really need what they think they need. It is like people in Southern California that buy 4 wheel drive SUV (Sport-Utility Vehicles) -- 99% of the vehicles never see a dirt road, rain or snow -- let alone a REAL need for the all wheel drive... yet they are popular (1). It is this mentality that the crowd claiming "the Mac doesn't have enough OS's" is really catering to -- they are trying to create FUD, but most people don't need to worry about it.
So the point is that the Mac runs one (two) really good OS's (MacOS and Rhapsody), which is what most people need. For those that need an alternative OS for compatibility reasons, most will not be disappointed with the Macs choices (and in fact many would be more disappointed with the PC's choices if they knew what they were missing). For those that want to run Alternative OS's for the "Hack" or "Geek" factor -- the Mac has Unix, BeOS, and other cool alternatives as well. It is near nonexistent that users will feel limited by the choices of OS's for the Macs.
Once again, the Mac comes out as a great platform that more people would chose, if more people were educated on what they were buying, and why.