For a while, namely in 2001, the Traveller was officially available without its case as well. The pen on its own cost about 375 euros then, and the travel case could be had separately for a little over 50 euros. Added together that came to more than the 400-euro threshold which Montblanc presumably did not want to show openly for this model. By 2002 the option of ordering the parts individually had been withdrawn again; the Traveller then cost 410 euros with its travel case, and as of 2005 the recommended retail price for the set was 445 euros.
When pen and case are sold separately today, the reason behind it is usually no more than a wish to sell two lots instead of one. For buyers that means a Traveller without its case is neither an incomplete nor a suspicious piece, but a set price for the pen alone is well over the odds. If you are putting pen and case together from two different sources, it is worth checking that the case matches the pen, since the leather goods were supplied in slightly differing versions over the years.