I DO see why. (Score 2)

There is no reason not to support all the possible customers you can if the cost to do so is low. As the number of users using unix os's at the moment is small (compared to Windoze), companies which do produce applications for the different unixen will most probably fall into two groups: those who also produce a version for NT/2000 (because there are just too many of them to ignore) and those who only produce unix versions.

To take the latter case first, the number of potential Mac OS X instances out there is close to the same number as of all other unixen combined. Those who produce only unix versions of software have the potential to tap into (say) twice as many potential customers, and the port will be much easier than porting to Windoze because they only have to deal with GUI issues (assuming there are any - e.g. porting cli stuff should be trivial), rather than GUI + Crappy OS + Different OS.

As for the former case, any company that has gone through the pain of porting to Windoze will likely have taken two routes: the dumb way, i.e. Branched their system entirely so that there is separate development of both versions and ne'er the twain shall meet; a smarter way, abstracted the GUI as much as possible from the logic. If they went the dumb way, the port will be harder and more expensive, so they may not see as significant an advantage - though it also means that they are doomed to waste lots of money as fashions in OS's change. The smarter way however, means that they can reap the benefit from their previous pain: the port will be much easier, and given that they seem to find some profit in their other unixen, the effort will be justified. Again; the GUI is the only real stumbling block: anything without a complicated GUI would be trivial, and OS X would just become another flavor of unix, offering a large number of potential customers.

The other potenial for crossover is for Mac Developers who have remained largely in that market to start developing (say) Linux versions of their products because they have already made the core logic of their products unix-ish. Such companies would essentially be just like the unix-only product developers; they get the advantage of a much larger market for their wares with a much reduced cost of porting.

In general, how couldn't there be an advantage?