A top can be for looks , but it does make a difference in tone depending on what and how its used. All NT US basses have solid wings (no other mfr to my knowledge uses solid 4/5A wings). That Spector growl that we all love so much? Every US Spector that I personally have played has that growl and tone acousticly before you even plug it in. None of the others I have owned (9) (except my US NS2 and my euro dougie) have done that. They sound good and raspy but not the same.
Most mfr's use a 1/4 to 1/8" top. The "Tops" on spectors aren't really a top and more of a body laminate. The maple on my euro is about an inch thick. About equal to the thickness of the alder back.
The difference in tone you'll hear is in the depth of the lows (that massive bridge helps) the pronounced mids "growl" and in the highs and in the attack. Believe me I've tried to copy that US tone with identical preamps hardware and pickups on other models . And wile they sound good and close their missing that "something". Maple should be the same as all other maple you would think, but its not. The softer maple used on the Korean models has a different attack, the highs are different (always), and wile they have good low end , they lack the depth and they just don't growl right. They do come close though (scary close sometimes). Some of the old NS2a's I've seen stripped have figured maple wings, not highly figured but Definatly decent looking flame.
The NSCR's I've owned & played (all maple wings) have been a little hit and miss. Always good, some I have played sounded like the Korean models (not a bad thing) and some were very close to the US models in the lows and mids but all always miss the mark on the highs.
They all sound great and pretty much like you think they should until you pick up a US NT bass. Then the differences in tone become quite apparent and definatly noticeable right off the batt with the first note. Acousticly. Plugged in it just gets that much better.
I shyed away from the new euro construction for a long time specifically because I didn't like Spector messing with the mix. After playing 2 side by side with 2 different US basses. I was sold. There's a lot more to the construction than adding a "top", and Stu didn't do it for $$$ reasons. The wood is where the tone is and you can hear it. Thankfully, Stu wants them all to sound like US basses I believe , and is a master craftsman enough to know how to do it.