I appreciate that I'm coming to this discussion fairly late but having read all the previous comments, my contribution is:
The Olympic class were designed with a focus on comfort, rather than speed. They were never intended to compete for the Blue Riband and so (travelling at a more leisurely speed of 21-22 knots), they were less likely (sea conditions dependant) to cause sea spray to such an extent that it was a sufficient nuisance to passengers on the forward potion of A (promenade) deck.
Therefore, the enclosure of the forward end of Titanic's Promenade deck, seems logically, to result from a desire by White Star to increase her tonnage, so that as 'the largest ship in the world', media coverage could be assured.
Equally, it seems logical, that the reason the forward end of Olympic's Promenade Deck was not enclosed, was because it would have made her appear far too similar to her ill fated sister.
Finally, the issue of progress in the design of ocean liner needs to be considered. Looking at those liners which followed Olympic i.e France; Imperator; Vaterland; Aquitania; Statendam (Justicia), it can be seen that the days of the open promenade deck were starting to come to an end. All these liners had a portion of their promenade decks enclosed. Possibly, this stemmed from shipping companies responding to the demands of their passengers for 'more comfortable' promenade space. Altenatively, the shipping companies were realising that by enclosing such areas with large windows, it provided the opportunity to creater larger public rooms; or provide additional facilities or simply boost revenue, either by adding more accommodation or by enlarging existing cabins which attracted a higher ticket price.
By the 1920s, the enclosed promenade deck was already a common feature on liners like the Paris and the Ile de France. As time progressed, ships that were built with a small portion of 'open' promenade deck (in the bow area or at the rear end of the promenade deck), often had this enclosed in a later refit. Example of this include the Britannic, Europa and Nieuw Amsterdam.
Sadly, in the post war period, the 'promenade deck' was to become an increasingly rare facility and if it survived at all, it was usually concealed as some form of shopping arcade.