What is the use of a clean OS (Lion) install? My machine is a iMac8,1 (24", early 2008) that I bought with Mac OS 10.5 Leopard installed. My question is: what are all the benefits and drawbacks of performing a clean operation system (OS) install? Specifically a clean Lion install in contrast to an incremental upgrade plan.

What I already know:


Upgrading to Snow Leopard and subsequently to Lion was easy and without loss of any data. I am aware of obsolete clutter being automatically removed in the processes (mostly PowerPC support), however, I can imagine that some is still present, e.g. Leopard and Snow Leopard system files that are now obsolete. They unnecessarily occupy space.
Another arbitrary advantage of a clean install has more to do with ‘personal hygene’: old (third party) programs that I haven't used (and associated files) won't be re-installed.
A third and last benefit of a clean install has to do with the physical space the OS occupies on the hard disk drive (HDD). Since the OS is practically the first thing on the disk it occupies the (faster) outer ‘rings’ of the HDD. That makes the OS faster than if it would occupy the inner rings. I presume that happens in the incremental upgrade as I did.


What are other advantages and disadvantages of a clean install? And are there any gaps or inconsistencies in what I already know?
There are no disadvantages to a clean installation. But the return on Lion has been severely culled due to Apple's decision to prevent any kind of modification to the OS. With previous OS X builds, users were able to select (and deselect) packages (such as printer drivers, extended languages, etc.) during the installation process. Lion brings everything and the kitchen sink. 

The ability to trim away parts of OS X was probably the most compelling reason to perform a clean install (saving GB of space).

Your notion of how hard drives work isn't accurate. There's no "prime" location for data, outer ring or inner ring. So long as the files are contiguous, your data will be delivered optimally. Access times depend on rotation latency (speed at which the disks spin, calculated in RPM) and seek times (the time it takes the head to reach the desired location to reach the file), although density plays a role too, by packing more bits of data per region (thus increasing throughput).

Moreover, OS X handles the defragementation of small files on a regular basis, ensuring continued performance over time. It's also of note that such things are gone by way of using SSDs.

Lastly, clean installations may also reduce bugs introduced by updates. While OS X is quite resilient, updates can introduce new bugs into the system. Often times, Apple technicians will request a clean installation and applying only the latest subsequent update in an attempt to isolate bugs (rather than upgrade from a previous version of the OS and then applying a slew of incremental updates). These problems are however quite rare.