DRAINING HOT WATER TANK?

I can't give an intelligence chemical discussion as to why you should replace the anode but I can give an  economical reason.   A replacement anode will cost somewhere around $15.00 and last at least 3 years ($5 per year) and can be replaced in about 10 minutes.  A replacement hot water heater will cost some where around $ 280 to $300 and take about an entire half a day to replace. Some how I doubt you would get 5 years out of a water heater without an anode but I could be wrong but guessing 5 years that would work out to be at least $56 per year.

Personnally I will take the former route and replace the anode.
The other point in favor of replacing the anode is the manufacturer wouldn't put them in if they weren't needed and save the cost . 
Now maybe one of our chemists can chime in and give us the chemical reason for replacing it or look it up in in  Wikipedia. Well OK I will.
 http://en.wikipedia.org/wiki/Sacrificial_anode#Choosing_the_Correct_Anode

I do know the anode doesn't coat the inside of the water heater but the reacted material is carried off in the water.  As stated in Wikipedia, the anodes used in fresh water are magnesium and in brackish water are aluminum and in sea water are usually zinc.

My undergrad degree is in Chemistry.  So I will offer an explaination that may be helpful.  Water has nasties that love to attack (chew holes in) the walls of your hot water heater.  The water being hot (at times) speeds up this attack. Think of the anode rod as CHOCOLATE YUMMIES  for the nasties  in the water.  They attack the anode rod, leaving our water heaters alone.  I hope this helps.    Rob

ps.  Either it's this explaination or the one with lots of long words, funny letter and number combinations, and guaranteed to put you to sleep very quickly. 

Haha Sandpiper,

My P-Chem professor frequently didn't show up to class on time. (I had to have four semesters of P-Chem too!)  So we would have to go find him.  We often found him talking to a wall, in one building or another on campus.  I think all those "+" and "-" had done him in too!    Rob

Thanks for the intellligent answers.  I think we will opt to replace the anode.  Next summer will be our 3rd year so will check it out at the end of the season.