Sure, there are many compelling reasons why you may want to retain the same lawyer who was involved in the negotiations and drafting of a particular employment agreement for a litigation arising out of a breach of that agreement, not the least of which is economies of scale - you won't be paying an outsider to get up to speed and familiarize themselves with the underlying agreement.
That said, tand putting aside for the moment that it may well be that your contract attorney isn't a true litigator, there are also a number of other instances where that option may not be viable, because it may well be that if there is an ambiguity in the contract and a sharp factual dispute as to the meaning of certain terms and what led to that final language the attorney could end up as a witness in the litigation - and the attorney can't be both an advocate and witness at the same time.
