Why deviation "however small" gives to the charterers right to treat the contract of affreightment at an end was never elucidated neither from practical nor from legal point of view. And after decisions in Joseph Thorley Ltd v Orchis SS Co Ltd  1 KB 660 it was never considered necessary. The rule devised to impeach a right of the shipowner to limit his liability hardened to the stone shape of legal maxim, and although it was operating in evident contradiction to the ordinary law of contract giving to the one side an option to throw up the contract even when contract was completely performed by the other side or when injured party suffered no loss at all, it was commonly accepted as a good law.
Per Tindal CJ in Davis v Garrett (1830) 6 Bing 716:
… We cannot but think that the law does imply a duty in the owner of a vessel, whether a general ship or hired for the special purpose of the voyage, to proceed without deviation in the usual and customary course.
Leduc & Co v Ward (1888) 20 QBD 475 per Lord Esher, M.R. at p.481S:
A bill of lading common mercantile document, which has been used for hundreds of years, and I think business men and Courts of Law have always interpreted it in one way, namely, that, if the only voyage mentioned is from the port of shipment to the port of destination, it must be voyage on the ordinary track by sea of the voyage from the one place to the other. So here, if the description of the voyage had been merely from Fiume to Dunkirk, I think the contract would have been for a voyage on the ordinary sea track of a voyage from Fiume to Dunkirk, and any departure from that track in the absence of necessity would be a deviation. Of course when I speak of the ordinary sea track I do not mean an exact line, for it would necessarily vary somewhat according to circumstances; the ordinary track for sailing vessels would according to the wind; the ordinary track for a steamer, again, might be different from that for a sailing vessel; I mean the ordinary track of such a voyage according to a reasonable construction of the term. In the present case liberty is given to call at any ports in any order.