some people don't enjoy playing the same story twice, just like they don't reread books or rewatch movies.
some people have very limited time to play and rather than reroll, they might just leave the game all together for something that lets them play what they do enjoy. just to name a couple. just becasue you cannot relate personally - doesn't make it invalid.
above was a suggestion to make it $20, limit it to 3 changes per character, and add a one month cooldown before you can change back. I'd say that takes care of "flash of the month" concerns nicely AND potentially makes bioware some profit.
For those that "don't want to do the same story twice" there are alternate means of leveling. True, they will have to do the story to get the story line companions, but this could be done once they reach max level and can faceroll the class story. There are two companions available without doing the story line. Between these two companions, any companion role could be filled, DPS, tank or heals.
Since there is NO time limit on leveling, everyone has the time to level a new character. It's not like you only have two weeks to level and your character is deleted if you don't get to level cap in that time, or that your character is stuck at whatever level they reach in those two weeks. Claiming "limited time" is simply an excuse.
If you think adding a limit of three class changes, one class change or any other limit would work, I would direct your attention to this thread and the countless others clamoring to get the current restrictions on class changes lifted or changed. I've said it before, but if you give a mouse a cookie...