Undoubtedly,it's offense the one that determines the outcome of a Pokemon battle,but it's equally quite risky focusing on strategies that involve *only* offensive techniques.A good example is probably Zangoose that has a quite commendable attack rate,but lacks severely in terms of defense-at least,in comparison to the rest of its stats-.Some claim that the ideal team is composed of Pokemon that thrive in attack/special attack,while defense/special defense is quite neglected.For instance,Dragonite owns mighty offensive abilities,but it's true it can be easily taken down,if stroke with Ice-type or Dragon-type moves(quite in fact,it's quite common to defeat a Dragonite by landing only one Ice-type attack).I really doubt that by training your Pokemon on an offensive scale solely,you get the win *ALL* the time,considering that on certain occasions you have to reenforce your defense in order to gain victory.It's evenly hard,nevertheless,to rely *only* on your defensive skills to achieve victory.Defense-based battles require patience(okay,a LOT of patience)in order to be concluded-and,especially,if the opposing Pokemon possesses recistance to your Pokemon's moves-,not to mention that it's wiser to teach your Pokemon mainly powerful moves that are unfortunately of lower Power Points and,worse,are of lower accuracy,say,Thunder.
Me,I prefer neither defense,nor offense.A balanced training is the key to win a battle,in my opinion.Besides,*nobody* wants his/her Pokemon faint without launching a single hit and *nobody* wants to waste precious time,attempting to defeat his/her foes.Though,I have a certain preference for raising my critical-hit ratio,so I usually give my Pokemon these sorts of items(Razor Claw,etc.).