It's not all about risk/reward scenarios, it's also about getting an even balance between teams.
Here's an example. On 'Midship' in Halo 2, you had an Energy Sword and a Shotgun sitting in the open in the middle of the map, in an exposed position where anyone attacking the area was vulnerable to fire from all directions. This creates satisfying risk/reward levels. However, both weapons were an equal distance from each team - from the start of the game, both teams had exactly the same chance to get these weapons.
Another example is the Gravity Hammer on Halo 3's 'Guardian'. The area it was located featured long, open sight lines and could be attacked from above, again creating a satisfactory risk/reward situation. It was also equally accessible to both teams from the start - Red team wasn't more likely to get it because they initially spawned closer or because it was plalced somewhere they could get to more easily than Blue team. You get the same situation with the Rocket Launcher and Sniper Rifle on 'Sandtrap', the Ghost on 'Isolation', and the Energy Sword and Rocket Launcher on 'The Pit'.
Reach doesn't follow these rules. On 'Sword Base', for example, Red Team can get to the Grenade Launcher completely uncontested from the very start of the game, as well as having a much easier time getting hold of the Shotgun. On 'Reflection', Red Team gets the Sniper Rifle and the Energy Sword straight away and don't even have to fight for them, while Blue Team have to fight if they want to get the Rocket Launcher. One team automatically gets the Grenade Launcher on 'Uncaged' and 'Powerhouse', and of course the Rocket Launcher on 'Pinnacle'.
So in short, it's not only that some teams basically get uncontested access to power weapons, but that they often get different numbers of power weapons too. It's not right that one team gets two free game-changers while the other team only gets one, and it's not truly a fun competition a a result.