Softcap Defense is good. Depending on your AT/Build/Powersets it can be VERY good, or good, or meh not bad. This depends on the type of Def you softcap, your play strategies, any synergies between your powers and that defense, and (of course) defense debuff resistance. It is NEVER bad.
Softcap Defense is a build goal. That will eat up valuable slots and choices as you place I/O enhancements. So the question becomes how much more resources you want to devote. The question should not be is more than softcap defense good. That answer is always yes.
The question is "Will adding more Defense past softcap to this particular AT/Powerset combo be worth the I/O enhancement investment?"
I would venture the response. F-ing rarely. If you are building a toon for pure incarnate content where higher than traditional softcap is necessary, you are almost always on a league throwing around defense buffs like crazy. After making the necessary I/O picks to get softcap defense you are probably neglecting a few other build goals that your AT/powersets might want/need. Should you completely neglect those build goals to chase an elusive number that may or may not be necessary? Maybe if you build a toon to solo incarnate content. That is the only reason I could think of. There are probably a few more. I am not massively creative until after I drop my 2nd or 3rd cup...