Until it breaks
Apple provides fixes for OS/X without the need to buy an upgrade. The paid OS/X versions are for complete new versions with additional software and functionality. This would be comparable to moving from Windows 2000 to XP to Vista in the Micro$oft world. So you are comparing apples with pears (or more exactly Apples with cabbages).
Then you are very lucky. In general, if a Windows machine is in heavy and daily use it is not capable of housekeeping itself. Registry management is but one example of that.
There's no need for anybody to do things to screw up Windows - it manages it by itself. Besides which, if it were a robust environment, it would not be susceptible to users screwing it up in their normal daily work.
That's an interesting challenge with Windows because much of the relevant information such as registry misbehaviour is obfuscated from the user.
All of which is untrue. It is perfectly possible to change things in OS/X. You do not pay extra for upgrades. For example, those for Tiger (10.4) are included and for several years. Purchased software such as Leopard (10.5) is a new software product with new features.