In Rotom, we have an Electric + Levitate + Ghost. If you replace 'Ghost' with 'Defenses' there, you'd have a Pokemon that would be extremely hard to take down. You'd be basically forced to give it a secondary type... which kinda defeats the purpose of creating a Pokemon going against the mold, in my opinion.
Because, really, you could say that Weavile, say, is a physical Ice type, but you could also say that Weavile is a physical Dark type. The former would go somewhat against the mold, whereas the latter would be completely stereotypical of Dark types. So which is correct? I'd say the latter would be correct. Look at Mamoswine. Would you say that its skewness towards physical is due to its Ice type or due to its Ground type? I think, undoubtedly, the latter.