@Seven: Yeah, the extra card draw means you have more things to spend mana on; most of the time I'm topdecking is when I'm on an aggro draw (and thus have lots of unbroken runes).
The flexibility argument is harder to counter. In general, early creatures are better than late for the mana, so that helps a little (especially in Arena, since most of the good big creatures are higher rarities). The Coin equivalent being three charges instead also helps smooth out draws (though it causes problems in Arena where going 2-3-4 gives a large advantage, especially in Solo Arena where the AI has a stronger deck and seemingly always gets the perfect curve, at least in the aggro decks).
@Dp101: You probably faced another beginner (assuming you went to ranked--no reason not to as you can't lose stars until 9, and in fact you can't lose rank at all (at most you go into a two star "hole" below the start of your current rank)), and thus didn't have a tuned deck. You can have themes though, and each color pair has a definite "default deck" it defaults too, like the Last Gasp synergies you noticed in Blue that also go with Green (I ran into that deck on the ladder once and it seems interesting, though the fact that one of its key cards is a non-unique Legendary that triggers all your Last Gasps is discouraging). The main reason you're focused on single-color at the moment is that you get three of each Ally during the Story, and three of the five are great (red and green are kinda lackluster compared to the others, and even purple is just an efficient body). There aren't that many mono-color incentives after the Allies though, so you'll see the benefits after getting more cards most likely.
The flexibility argument is harder to counter. In general, early creatures are better than late for the mana, so that helps a little (especially in Arena, since most of the good big creatures are higher rarities). The Coin equivalent being three charges instead also helps smooth out draws (though it causes problems in Arena where going 2-3-4 gives a large advantage, especially in Solo Arena where the AI has a stronger deck and seemingly always gets the perfect curve, at least in the aggro decks).
@Dp101: You probably faced another beginner (assuming you went to ranked--no reason not to as you can't lose stars until 9, and in fact you can't lose rank at all (at most you go into a two star "hole" below the start of your current rank)), and thus didn't have a tuned deck. You can have themes though, and each color pair has a definite "default deck" it defaults too, like the Last Gasp synergies you noticed in Blue that also go with Green (I ran into that deck on the ladder once and it seems interesting, though the fact that one of its key cards is a non-unique Legendary that triggers all your Last Gasps is discouraging). The main reason you're focused on single-color at the moment is that you get three of each Ally during the Story, and three of the five are great (red and green are kinda lackluster compared to the others, and even purple is just an efficient body). There aren't that many mono-color incentives after the Allies though, so you'll see the benefits after getting more cards most likely.