Mind you, if anything, it should be considered "poor form".
Old MTG:P used to have two players on the same side using the same deck "blocked". (There were some exploiters who used a workaround, but it was still illegal). With the introduction of 2012, I see alot of unskilled players using the same deck to try and get ahead in matches.
Mind you, using the same deck can cause more harm than good in some situations... but, more often than not, I see it being more of a cheap tactic that succeeds, especially in ranked matches.
I think that's where my concern lies... In ranked matches. I don't really care what happens in unranked ones. If some twerps play same deck, I'll either drop and move on, or just hit 'A' til its over. Well, that's not true, either. There's no FUN in playing against unskilled players looking for any unfair advantage they can get. Its amusing when they fail, but in those cases, it was either so unchallenging as to be a joke, or so ridiculous that I wish I didn't waste my time.
So, Wizards... If you read this, and if you agree, re-block same deck 2HG matches. At least for ranked matches.