Rataan:
Not at all. Defense Ships still win. Against anything less than maximum fleet power (and there can't be too many of those), your Defense Ships can handle them handily. If the attack edges higher, include one all-attack ship with a mostly Defense fleet. The purpose of the Attack Ship is to exchange with one of theirs. The exchange is in your fleet's favor because it lowers the attack of their fleet below your defenses.
But if you are content to trade ships, then defense is irrelevant. In fact, you can often manage to not lose ships even if you don't have any defense at all, simply by choosing your battles such that your opponent's attack values are less than twice your hit points.
I had a brutal (and fun) war with the Drengin at crippling difficulty pre-patch. They won the colony rush and were better than me in every category and were running away. Being Drengin, I was at war with them in as soon as the colony rush was over, and it looked like they were going hurt me...bad. We both built all attack ships, although theirs were much better than mine. Still, I was able with Eyes of the Universe and better speed to muster as much force as I needed to stop their transports, which were always heavily escorted, and I never lost a planet.
I couldn't keep up with their weapons for a long time, but what I did better was to concentrate on larger hulls. We went to mediums at about the same time, but I switched to large and turned the tide of the war. By the time they were building significant numbers of large hulls, it was too late. I was starting to roll over them. They had lots of medium ships with 140+ attack, but that wasn't enough to take out a large hull in one round in most cases. Even if it was a tie, the 1 hp rule would usually be in my favor. But my large hull could take out two medium hulls in one round. I battled several fleets with 350+ attack, but it wasn't enough to destroy a fleet of three of my large hulls. I would win with at least one survivor, usually two, even though my attack values were much lower.
If that sounds like heavy losses on my part, remember that I was taking out some very dangerous and expensive ships, and my losses were substantially smaller than theirs. And unless a ship has very high experience, it is usually better to build a new one than upgrade an old one. Newer ships are often cheaper to build and maintain in additon to being more powerful. This was a good use for aging designs that didn't have much experience.