Is Master of Arms pointless under new blocking rules?
You are correct, that Master of Arms ability is basically useless now, and functions differently now than it used to under pre-6th Edition rules.

If there was errata for Master of Arms', it would show up in the Oracle wording. The Gatherer page for that card lists the Oracle text, along with errata changes and dates. The Oracle is the official wording on Magic cards. The card used to work the way that you described, but in May 2011, the errata that supported the cards original functionality was removed so that the card now works as written.


  Master of Arms - Just like Winter Orb, Master of Arms had a mystifying Oracle wording intended to replicate original functionality. The rule that changed this time was the one saying that tapped blockers didn't deal combat damage. ...
  
  My general philosophy is that I'm okay with rules changing out from underneath cards and changing their functionality. Heck, just look at the Magic 2010 rules changes.


This recent change to Master of Arms is part of an ongoing rules philosophy change that began years ago. Several different articles point to the reason for these changes. In Power-Level-Errata-B-Gone, you learn of the architect behind the rules changes, Mark Gottlieb the lead rules manager. The reason for the change is that without an online reference, it is difficult to verify how a card is supposed to work. For that reason, power-level errata is being removed from cards as well as errata to make cards work as originally designed.


  Why Did We Make This Latest Batch of Changes? - Over the years, the sheer number of rules revisions and changes of rules managers the game has undergone—as well as unintended consequences that arise when new mechanics and card abilities are unleashed each set—has led to cards slipping through the cracks. As the evil genius currently in charge of maintaining Oracle, Magic rules manager and former cult-leading columnist Mark Gottlieb is a strong, strong believer that cards should have functionality that matches their printed intent as often as possible.


Ben Bleisweiss talks specifically about tapped creatures and combat damage under fifth edition rules (as well as other interesting changes). The rule was essentially removed, because it wasn't intuitive to new players. Someone who was unfamiliar with the rules probably wouldn't understand why tapped attacking creatures deal damage, and defending creatures don't.


  TAPPED CREATURES DEAL NO DAMAGE - Change: Under Fifth Edition rules, tapped blocking creatures dealt no damage in combat. Under Sixth Edition rules, they do.
  
  Why: Much like the old "tapped artifacts shut off" rule, the rule about tapped creatures dealing no damage wasn't something a player would intuitively know just by looking at a card.


Mark Rosewater even lists Master of Arms' as a famous example of cards being useless under new rules changes.


  In fact, our Oracle database encounters this problem every day because it has to modernize cards and mechanics that were created in a different time. Often what was done then doesn't even work now. I don't know how many times I've heard Mark Gottlieb gripe because some card that was made to do something that made sense in the past simply doesn't make any sense in modern day.
  
  A famous recent example would be the card Master of Arms. The card was designed at a time when tapping a blocking creature kept it from dealing damage. Then Sixth Edition rules came along and removed the rule and all of a sudden Master of Arms's mechanic made a lot less sense.


All that being said, Master of Arms isn't really a tournament level card, which means that you are probably playing this deck in a casual setting. Most likely, if you told your opponents, "Hey, I have this fun deck I play that got completely hosed by sixth edition rules, would it be ok with you if Master of Arms said '1W: Tap target creature blocking Master of Arms, that creature deals no combat damage this turn.' for the purposes of this game?" I don't think anyone would object.