Due to the highly competitive nature of air travel in the US, the 757-300 is only useful on those rare routes where schedule-frequency is not important. Examples of such routes are:
1) Heavily travelled leisure-corridors (ie...a big city to a major resort)
2) Medium/long-haul, high volume routes.
Now, let's look at Alaska's route system, and you will see why the 753 doesn't make any sense.
Firstly, the 753 actually has LESS range than the 73G. So, it's not like the 753 would allow them to serve markets which they can't already serve. Also, the 753 would add a new type, meaning additional training and maintenance costs. But beyond that, there just aren't any routes for it to serve. If you don't believe me, let me give you a closer look at Alaska's route system.
Alaska (excluding Horizon) has five main types of routes:
1) Flights up and down the West Coast.
These are predominantly medium distance, well-travelled, competitive corridors. On such routes, it is neccessary to provide passengers with a comfortable, fast aircraft (ie...not a turboprop), but also critical to tailor to the schedules of business travellers, and to allow fast boarding and maintain good gate utilization. An aircraft such as the 737 serves as as good compromise between these conflicting demands.
2) Intra-Alaska markets.
With a few exceptions, these routes are medium length and low-volume. Alaska has squatter's rights in this market. Other airlines avoid the state because it is too far away from the rest of the US (hence the rest of their route network) and also because the operating conditions are very difficult. All Alaska has to do to protect their monopoly is to satisfy the demands of the communities they serve (in my mind, provide acceptable schedule frequency, charge prices that are just low enough to avoid an outcry, and offer modern jet equipment). Not only is the 757-300 not needed on these routes, it can't even get in and out of some of the airports.
3) West Coast to Mexico. This may seem like a good 753 route, as it is a large leisure-travel market. However, the potential for connecting traffic makes the idea of running planes back and forth between Mexico and the US much less compelling. Alaska's goal is to funnel passengers to SFO and LAX and then fly them to Mexico. Because the planes that fly to Mexico continue on to non-753 friendly routes, and because connecting traffic makes schedule frequency more important, a 737-sized aircraft is once again the logical choice.
Any flights Alaska offers between SEA or PDX and Mexico are purely designed as a defensive measure to keep other airlines from serving the route and stealing customers. Therefore, Alaska wants to use their smallest plane...the 73G.
This is a route in a class of its own. Alaska has a monopoly on the daytime flights and it is also a very high volume route. But, again, the route depends heavily on connecting and one-stop traffic. A typical plane serving the SEA-ANC route might originate in FAI and fly south to LAX.
Again, Alaska's goal is to funnel passengers into the SEA-ANC corridor. The PDX-ANC, SFO-ANC, and LAX-ANC routes are once again defensive.
5) Long-haul, West-East routes.
So far, the only routes are ANC-ORD and SEA-DCA, but there are rumors that more are on the way.
As Alaska is a newcomer to the trans-con market, they have to look for underserved routes. In general, any route where a 753 (or other high-capacity craft) could be used has already been fully exploited by other carriers. So, Alaska needs to use smaller aircraft on smaller routes. If Alaska can successfully enter long-haul, high frequency markets, then the 753s could make sense. However, I doubt this will ever happen, as many of Alaska's valuable airline partners already fly these routes. So, if Alaska continues eastward expansion, they will look for underserved routes that have the pontential for a lot of feed from airline partners.
So, I think Alaska should just stick to its current fleet plans.