I just had this hit me yesterday (PS4/multiplayer). This is extremely unfortunate.

I had just socketed a superb rune in this bad boy and I was excited to put it beyond the level of my Mindchill Staff ... just to find out I couldn't upgrade it with a grip and blade?!
Also - all the theories about max upgrade slots ... I don't think they apply. My Blizzard's Path not only does "explode for 75% weapon damage" as well as a couple other effects - it also allowed me to add a grip, blade, and rune - it has a list of like 9 or 10 additional effects. Unfortunately it is low dps level 12 unique - hence why I was trying to get this level 18 working.
My 2 cents ... I disagree that uniques should necessarily have more slots - especially if their native bonuses are good and the UI is clear and not buggy. Also, I've received both types of weapons for all rarities - ones with just rune slots and ones with all slots.
The real kicker is just the fact that there are weapons that have this bug that really make it disgruntling - regardless of server or UI issue - especially if you've burned a good rune like myself on such a weapon. Here's to hoping this gets fixed quickly ... (along with the issue of keys failing to drop)
One more thing ... though it may not have been best judgement to post a private message - I equally find saying "... to avoid inevitable amateur design discussions" is not in the best taste - we don't really know anyone's backgrounds here. But there is one thing we can all agree on - we all would like to see Dragon Age Inquisition be the best it could be - either because we're working on it or we love playing it.