This is the same excuse that Douglas Carswell gave for resigning from UKIP recently and it still doesn't hold water. Article 50 may have been triggered but this is only the beginning and UKIP is about so much more than getting us out of the EU.
Like Douglas Carswell, Mark Reckless is refusing to resign and call a by-election which they both insisted was the honourable thing to do when they defected to UKIP in the first place. The people of South Wales East voted for two UKIP AMs off the regional list, they didn't vote for Mark Reckless. The right thing for him to do is step down and allow the next person on the regional list to take his place.