Here is a good example
I was looking to fly from SJC-ABQ. US Airways wanted $313+Bag Fee. Southwest wanted $243.00. United $358, Delta $350 and United $320.
Now If I fly to New York I would go on United , Delta for $280-$300 since they offer non-stop flights and PTV on the longer routes. I also don't mind paying for buy on board meals. Considering I can take the redeye it works.
For flights from SJC-ABQ there is not a whole lot of options and good timings from SJC. United and US Air have limited schedules and due to the probability of snow I booked via PHX in December and WN had the best fare. Sometimes US Airways is cheaper and if US Airways and WN are $10 difference I book WN for the free bag rather than carrying it around. If US and WN were $50 then it would be US Airways and I would pay $25 for my bag.
Now from SJC-LAX I prefer Horizon Air since I can access the Alaska Lounge and then its a short trip where checking bags are not a priority.
At Phoenix with my Priority Pass I can go to the US Air Lounge so no problems there.
All in all WN works on the short haul 1 change or one stop flights but not on the long haul flights with 2 plane changes+1 stop. I would much rather carry my suitcase on to take the nonstop SFO-JFK flight than wasting 12 hours to get to where I am going.