It's still a good game for $4, but unless you really want to play some specific mods for Mount & Blade that aren't available for Warband, then you'd just be wasting your money and would probably never play it.
Warband is the same game as Mount and Blade but with more features and there's enough content in the plain unmodded Warband to keep you entertained for months... then there are Warband specific mods that will increase your enjoyment even more, so chances are you'll never really play Mount and Blade.
The original Mount and Blade is good, but is inferior to Warband in almost every way and as a new player, I don't think you'd appreciate or utilise anything in the original Mount and Blade, so it would be a bit of a pointless purchase.
It always going to be cheap now that Warband is out, so wait until you've become engrossed in the game and find a reason to want to play the original, to see where it started or try some of the older game play.
Wait until you don't have to ask this question because you know you want to try something that only the original Mount and Blade can offer.