This technet article http://technet.microsoft.com/en-us/library/cc263445.aspx describes the accounts required for a SharePoint 2010 installation. I've always followed this and my clients have never had issues creating so many accounts for a SharePoint installation.
Now this one client has a network administrator who wants to make do with the 3/4 accounts he's already got created for some other SharePoint installation done by a different vendor.
He's argument is that it reduces the number of accounts that must be managed as he is the only one looking after their network and infrastructure.
I think that once created, these accounts don't need to be managed at all and if they do, then this can be done via Central Admin managed accounts section.
Can you guys let me know what the drawbacks are of using one account for say the search services and search content access?