Balance does not mean that all weapons are equal in all situations. Rather, it means that weapons will be good in some situations and bad in others, but overall, will be fairly equal.
Very true, and this goes hand-in-hand with the map you're playing on to account for the weapon balance as well. Maps are an important factor in your weapon choice. On a map like Tropiccave or Nuubia, there isn't much room to dodge an M79er, and we are all aware of this. You're just going to have try and be faster or prone and jet as soon as he fires, and even then they can hit you if they're a skilled player. Hey, it's a small map. However, on a map like Shau or ctf_Run, there is a much larger area for you to maneuver, which means the M79 can literally fall short. The only exceptions to this is either the M79er sneaking up behind you or both you and the M79er running/skipping full speed towards each other, and when you see each other you'll be in a close enough range to be hit. Otherwise, he's easy picking.
Obviously the M79 and Spas will do much better on the aforementioned Tropiccave or Nuubia, but you will probably have to switch to a Ruger or Minimi on a map like Cambodia or ctf_b2b.
It's all relative, people.