Well here is a scenario.
This assumes that you could resell at 80% every year. Also assumes you are buying about the same level machine each time - in this case a $1499 13" MBP.
Bob purchased a 13" MBP (actually it would have been a aluminum MB then) for $1499 in 2008
Bob kept it for ~4 years and then sold it for $600
Bob purchased a new 13" MBP in 2012 for $1499
$1499 - $600 + $1499 = $2398.
Fred purchased a 13" MBP for $1499 in 2008
Fred sold the 2008 MBP for $1200 in 2009.
Fred purchased a 13" MBP for $1499 in 2009
Fred sold the 2009 MBP for $1200 in 2010.
Fred purchased a 13" MBP for $1499 in 2010
Fred sold the 2010 MBP for $1200 in 2011.
Fred purchased a 13" MBP for $1499 in 2011
Fred sold the 2011 MBP for $1200 in 2012.
Fred purchased a 13" MBP for $1499 in 2012
$1499 - $1200 + $1499 - $1200 + $1499 - $1200 + $1499 - $1200 + $1499= $2695
So in the end both Bob and Fred have identical machines. However Fred paid about $300 more than Bob, so yes in this case it did cost more. But Bob always had the newest technology AND a warranty.
Now lets say that in 2010 Bob had an issue with his then 2+ year old MB and it cost $300 to replace the logic board. So in this case they spent the same and Fred still had the latest and greatest every year.
For some the additonal $300 over 4 years is well worth it. Now maybe 80% resale is a bit high, or the resale of the 2008 at $600 is a bit low but I think it is in the ballpark.
I used to always have a desktop and a laptop and I'd replace each every other year (alternating years). This kept my urge to update constantly at bay by sticking to such a system. However I stopped buying desktops when Apple changed their philosophy and really got rid of a standard desktop (not a fan of all-in-ones, mini underpowered, and the Pro just kept creeping up in price point). But that is a different topic. So now I typically get a new laptop every 18 months or so.
How come neither fred nor bob paid tax? Given a modest 5% tax, Fred would have paid a further $300 more. If you put in much more realistic numbers (75% back, and tack on tax) you'll see the margins are MUCH larger.