The BNP's backing down indicates that they would not boycott elections at all, even if they were held on the 18th as scheduled. Delaying the polls by 10 days really does not benefit the BNP. The rescheduling of the dates for nominations gives them a little additional time to prepare, but as far as the polls are concerned, 10 days will make absolutely no difference to their preparation or chances. The only logical reason that I can see for this demand is that delaying by a week to 10 days puts the elections during Christmas time. Many foreign observers will cancel their trips. I'm sure you are aware that there are still a fair number of BNP sympathizers within the administration, especially the EC, etc.
My concern is that the BNP may still be hoping to make an attempt, however unrealistic, to act as the spoiler. Besides, shifting the elections introduces uncertainly into the whole electoral process again. Right now they are set and everyone excepts that the BNP is prepared for the polls. It is simply not right for the BNP to be playing around like this, especially when we know they were bluffing all along!