The gap doesn't exude a "quality" look and feel to the unit. The installed DVD drive doesn't have a gap so if the gap was to be a "good" feature, then why was the Enhanced Ultrabay DVD drive released? Certainly the 9.5mm Slim Ultrabay DVD drive could have been used, correct (it does fit)? No, this is about cost tradeoff and not about fit and finish. The third party adapter uses 4 screws to attach the drive to the adapter which makes it quite secure as well. The bezel matches the lower case perfectly. Maybe there will be an "official" Enhanced Ultrabay SATA adapter released in October.
And one other thing: Is there a Ultrabay Sata adapter which hasn't a gap for my w520?
I have the Lenovo adapter and have had no problem. The HDD stays in place with the locking bar in the adapter, and the adapter itself is stable --- in fact it fits better than my battery . A lot has been written about this adapter, but bottom line I have had no issues (at least until the dog hair starts flowing through that gap). And after using a T61p I am happy to have as much air as possible flowing through my machine.