How are defences worthless? There's only an additional 12% apen and 5% mpen on Dominion. Since that's multiplicative, your final apen total after a Last Whisper the the 10% mastery is not dramatically higher that it would be on SR. It's obviously even more irrelevent for mpen.
The only thing I will agree with is the cost of advanced magic defence items are too high for Dominion. Items like Veil, FoN, and Abyssal generally take up nearly a third of your total Dominion gold income, and are rarely built. People just usually slap on a plain Negatron, or get something hybrid like Wit's End, Hexdrinker, or Aegis.