Hm. Well, the best answer I can give is that your class is very temporary, and your abilities are (mostly) permanent. If you spent a lot of credits on Engi abilities, you can still switch to Ops during a game, but then your character wouldn't be able to use the abilities you bought (unless you also have a nice reserve of Universal abilities).
The Brink Bible mentioned that there is (currently) a 24-hour grace period for ability purchases in which you can sell the ability back for a small cost if you don't like it, with the cost escalating after 24-hours.
This is a good reason to utilize multiple characters -- that way you can specialize and tailor each one.