Would it be useful to set a partition on my internal HD only for data and the other for apps, OS and that kind of stuff?

Question

As I wrote in the title, would it be useful?
I come from a Windows PC where I had that kind of division and it seemed everything had worked fine along 5 years.
Please, give a motivation to your answers to let me get in the point.

Answer 1

File fragmentation is not an issue with OS X, which looks after itself.
Defragmentation in OS X:
http://support.apple.com/kb/HT1375  which states:
You probably won't need to optimize at all if you use Mac OS X. Here's why:
Hard disk capacity is generally much greater now than a few years ago. With more free space available, the file system doesn't need to fill up every "nook and cranny." Mac OS Extended formatting (HFS Plus) avoids reusing space from deleted files as much as possible, to avoid prematurely filling small areas of recently-freed space.
Mac OS X 10.2 and later includes delayed allocation for Mac OS X Extended-formatted volumes. This allows a number of small allocations to be combined into a single large allocation in one area of the disk.
Fragmentation was often caused by continually appending data to existing files, especially with resource forks. With faster hard drives and better caching, as well as the new application packaging format, many applications simply rewrite the entire file each time. Mac OS X 10.3 onwards can also automatically defragment such slow-growing files. This process is sometimes known as "Hot-File-Adaptive-Clustering."
Aggressive read-ahead and write-behind caching means that minor fragmentation has less effect on perceived system performance.