Harrington,
The problems that came about from two tickets was because there was no understanding between the two or more airlines on what happened when something went wrong. While most airlines simply applied the IATA rules others did not, perhaps only when it was in their interest to do so.
I think it was BMI who started refusing through check with two tickets because they had been 'stung' by a few interline partners. (The IATA rules say the receiving airline settles with the passenger and then bills the delivering airline for a share of the cost. They were paying the full claim and had their bills refused because there was no contract)
It would seem that one world airlines do have an understanding for multiple ticket interlining, so you should be OK if something goes wrong.
Tableview - another reason is the network airlines use the 'full Y' fare for other things than just the very rare one-way traveller. Excess baggage charges, ship's crews discounts, etc etc. They still use them, in some cases, when calculating shares from an interline ticket. Where they have been forced to have low OW fares to remain competitive, they will introduce these in addition to their 'full Y' rather than reduce this fare.
Radeng - As Alaska is not a one world carrier I would be careful using separate tickets. Check first. BA phone sales (or a travel agent) should be able to issue any possible combination of fares on a single ticket, and while that may add a booking fee you are always better off with a single ticket.