Far from silly - "playing" with a BB gun has made me an infinitely better shot than I ever could have been shooting a .45 til my funds ran out. Walmart of all places sells a $29.99 1911 officer's frame BB pistol which will put 15 rounds in a quarter at 8 yards. I've worn three of them out over maybe 20,000 rounds.
Total expenditure on guns: 90 bucks
Total expenditure on ammo: 30 bucks
Total expenditure on CO2: 150 bucks
Equivilant cost of 20,000 rounds of .45 alone: $5,000
You'll still want to shoot your firearm regularly to learn the art of ignoring recoil (and because "boom" is more fun than "pop"), but trigger control and sight picture can be practiced much cheaper. I also became much better at point shooting after a few thousand tries. It always seemed like a bit of a waste to draw and fire instinctively at the range only to see the pristine target laughing back at me. That is no longer an issue.
My backstop is a two foot by two foot 2x4 frame with a 7/8" plywood back with several layers of old rags & toweles stapled over it. BBs tend to pentrate a few towels and stay in the backstop - your mileage may vary with airsoft but I suspect the same setup would work.
Be aware that Airsoft blowback guns will rip through CO2 at an alarming rate as opposed to fixed barrel models or BB guns. I had the displeasure of shooting a 1911 gov't size airsoft replica - it field strips like the real deal and the slide functions, but you have to replace the CO2 cartridge every time you fill the magazine (15 shots). It is also worth noting that Airsoft is largely marketed towards kids and thus one of the primary selling points on many of the guns is that they are "heavy". Fit and finish don't come into play; I think you just have to get lucky to get a good one.
Sharp as a cue ball...but moving faster.