As for the 'dirty' bit of actual physical conflict - I think it's better to train to stop fights, as opposed to training to fight. I don't want to get into a situation of drawn out blow-trading.
From the Taiji perspective, I want to absorb my opponent into my 'cloud', where I'm going to take him apart very quickly. So, I train to draw in the opponent's attack, move out of his line-of-intent, into one of the outer gates, or to the rear, where striking to vulnerable points is easiest, before throwing or tripping him (painfully) to the ground, at which point I will hopefully be able to make an escape.
The aim is to actually 'hit' the opponent no more than half a dozen times, and strikes are used to set up the 'big drop', which may then be followed by a 'finishing' strike.