Having a ranged option is nice, but being able to have Charge available at all times (for shield recharge) is FAR more important. The Nova/Charge synergy is so incredibly powerful that there are very few situations to remain at range (upload objective and extractions are the only ones where I sit still).
Shockwave upgraded will lower the cooldown on all your powers and it's not always a good idea to charge into an enemy. It gives you a ranged option.
Maxing all the skill trees is also only possible if you put *zero* points in Shockwave. Why is this important? Why not sacrifice maxing out one skill if it means you can put a few points into Shockwave? Because of the combined synergy of all the other skill trees:
Charge: Level 6 gives you 100% shield recharge instead of 50%. Besides the added protection, this allows you to use Nova to its fullest capabilities.
Nova: Level 6 gives you Pierce, which does +100% damage to barriers/shields/armor. Absolutely key in taking down the most dangerous enemies in Centurions, Phantoms, Atlas' and Turrets. (Also level 5 gives you the ability to only use 50% shields per use. Even if your shields are at 55% you can still use Nova twice, which keeps you invulnerable long enough to immediately use Charge again).
Fitness: Level 6 gives you 25% increased health/barrier bonuses. Besides the extra life (825 each of barriers/life), this means your Nova is a) more powerful and b) more likely to be able to be used twice between Charges.
Assault Mastery: Level 6 gives you a power damage bonus of 15%, and power duration/force bonus of 25%. You can go entire rounds without firing a single shot with the Nova/Charge combo. You can solo an Atlas (in his FACE) with this combo. And giving a bonus to both damage and force means you can kill everyone faster. Why does the force bonus matter? Well, even if your shields are at 5% you can still use Nova. Why would you? Because if the enemy doesn't have protection, then they will at least stagger, if not fly up into the air outright. This gives you (and your teammates) tons of breathing room when 3 guys all hit the deck at the same time, making them incredibly easy targets (and not able to shoot back at you). And note that with level 6 Nova (double damage to shields/barriers/armor), dropping their protection is easier as ever. With this build, if you have full shields a Nova at close range will immediately drop a Phantom's full barriers and stagger them. The second Nova will make them fly into the air, and a Charge will usually finish them.
What does Shockwave give you? It's solid at range, and it gives a way to stagger Guardians. But both Nova and Charge stagger Guardians as well, with the added bonus of being at a *consistent* range when you do so. It becomes second nature to Nova or Charge at close range, then immediately go for the headshot since the distance will be about the same every time. This isn't true with Shockwave. If an enemy is close enough to use Shockwave, it means they're also close enough to run up to and use Nova. And a (human) Vanguard at range is a Vanguard not living up to her capabilities. Also, using Shockwave means you *can't* use Charge, which is your lifesaving skill.
And yes, I agree, it's not a good idea to go Charging everywhere. This is why I've advocated in all my posts to lead with Nova, *not* Charge. Leading with Nova means you have time to survey the situation (ie, look for turrets) then Charge to get your shields back if you need to escape. Leading with Charge means you have to wait for the cooldown, which is death on Silver and Gold.
Edit: On top of everything above, favoring Nova over Shockwave means you don't have to aim your Novas (since it hits in a radius around you). Lots of times I'll run blind (literally if there's a smoke grenade out) into a hotspot and just lead with Nova. If my shields drop early then I'll have Charge available to reset (and escape if need be) but if not then I'll go with a second Nova to weaken enemies further. Clean up with a shotgun or pistol or melee if there are stragglers or go back into Nova if there are still a lot left. Point is you don't have to aim if you know you're surrounded.