I make great use of Sandbox for testing various engineering techniques (like the buff trick that needs to be fixed so badly). That's where I developed my Squid engine repair circuit (which makes Squid engineering way more fluid, and way more fun).
Indeed, the problem I feel with this and the other achievements is that you must be in sandbox with players below a certain level. Whenever I've used sandbox, it has been to practice
MLG PRO NAVAL FORMATION META STRATEGY attack angles on different weapons and some minor manoeuvres with higher level crews.
(sand box is lacking at the moment with only one map and one ship, it could do with allowing multiple ships, multiple maps, and the ability to spawn ship models in place of the target dummy's.)
New low level players tend to learn everything they need from the tutorial (we wish... allot of them, myself included, tend to go straight into pub matches with no idea of what to do) then jump into the game and level up.
New players want to actually play the game not just sit in sandbox. the main benefit of sand box for teaching inexperienced players occurs when they have already gained a few hours legit game play, by which point they usually have levelled up. The most valuable experience comes from actually playing the matches.
EDIT: In addition, I feel that the time requirement for you to spend in sandbox is unrealistically and unnecessarily high, everything a player needs to gain from a "lesson" in sandbox can be completed within 20 minutes - 40 tops.