Yes.
A good programmer would do this. Regardless of what the designer thinks the client SHOULD be doing, there's no telling how the client might decide to try to do things anyways. Why should a simple calculator block any input that isn't a number? Do people really try to add Z + leet * awesome? Well, yes. They could. And it would break their calculator. So even though adding words is retarded, the programmer has to allow for completely retarded/unanticipated input and make sure to restrict it from screwing up the program.
In an ideal program, you only allow exactly what you want to allow through, and block out everything else. If you only want your inventory to be equipable, you add catches to ensure that items in any other area cannot be used as equipment. I would be sincerely disappointed if SE's professional coders were so lax in designing fundamental code for use in a worldwide MMO.
I'm not even really sure what he (DefiledSickness) is even trying to say here. Though personally, even if I had implemented a block/catch with only one or two lines to prevent equipping items from non-Inventory areas, I would still have to do more than just delete that line. If I had a restriction in the first place, it was for security reasons (anti-exploit). Simply removing an anti-exploit measure and letting that be that would be an enormous breach of security. Even after removing the potential limitation to inventory-only equipment, they would have to redesign the code in such a way that players are able to equip from 2 sources but unable to find ways to trick the server into letting them equip from other sources.
It is just plain dumb as a coder to assume that either: A) No client is smarter/more savvy than you are, or B) No client will intentionally try to break/exploit/hijack your code. If the possibility exists, someone will pursue it. Taj made that abundantly clear when he still played.
Ironically, this is also a simplistic security fix. There used to be no delay in moving items, and people exploited this in order to dupe items in their delivery boxes and other systems. They could remove the limitation, but that would require a significantly more sophisticated anti-cheating fix to the code which I highly doubt they'll pursue.