Deleting old preferences, any pros or cons?

It depends on what apps you have. Preference files contain information for the preferences in various applications. Deleting that file resets the preferences in your apps they are for. In order to identify which file you need and which file you do not, the format for a preference file is as follows:

com.(company name).(application name)

Delete those preference files that do not have any apps associated with them in your Applications folder. There will not be any major speed bumps though, maybe there are minor ones.

deleting preferences doesn't increase speed but it might improve performance of some programs if there are any corrupted or conflicting preferences. a simple test is to move the preferences folder to the desktop and log out/in. However, I would not chuck the whole folder without need. some apps keep important user configuration data in that folder. for example mail keep the info about all your email accounts in its preference file and if you delete it you'll have to recreate all your accounts in Mail. but generally, deleting user preference files is safe and simply resets them to the default state.

Message was edited by: V.K.

Correct.

Steve Sherriff1 wrote:
So the preference files are not looked at by the OS until the program in question is opened?

yes.

If so I guess I am safe to leave them in place and it will cause no harm.

yes.