Like Diablo, Warframe is a game about acquiring better gear. Unlike Diablo, the loot tables are static and, for the most part, known. "Gear" in Warframe == mods. If you set out to optimize your frame and weapons then you know exactly what mods you need and exactly where (and how often) you can get them. It's a waste of time for me to spent 30 minutes killing everything on some Rescue mission when the only reason I'm even doing the mission in the first place is to unlock one after it: a Survival mission that was carefully selected because the faction is Infested and it's Tier III. I need that mission unlocked so I can grind it repeteadly for the 6.67% chance that a
Flow mod is given for the 20-minute reward tier.
This game is about optimizing your gear, and the most efficient way of doing that is through Survival and Endless Defense missions. I'll get
at least twice as much XP and three times as many mods/Fusion Cores as you if I spend my 30 minutes on a Survival and you spend yours clearing every enemy and opening every container on a Rescue. Plus I have the chance to get one of my "optimal" mods at 20 minutes! Win/win/win. That Rescue mission is a mere hurdle,
not something to be treasured.
As for Alerts, it's all about the rewards. There are many items that can
only be acquired through alerts (or platinum, obviously). Orokin
Catalysts and
Reactors (aka "potatoes") are essentially required for every endgame weapon/Warframe and they only pop up as rewards for random Alert missions. During a good week there might be 2-3 chances to get one. Hopefully you catch the Alert (they can occur at any random time, 24/7/365) or have the greatest lucky streak of all time and acquire a bunch through daily login rewards because those are the only ways to get potatoes and you'll need
a lot of them if you're like me and enjoy switching up your weapons and using different Warframes. The best Aura mod in the game can also only be acquired as an Alert reward. It randomly pops up about once every two weeks on average. The Warframe helmets are another good example. Those seem to pop up just about bi-weekly as well, and if you don't want to spend 75 platinum apiece then Alerts are the only way to get them.
Nightmare Mode is also excellent because completing a mission awards you a guaranteed Rare mod that can only be earned in Nightmare. The Nightmare mods are great because they all have two affixes, both positive (
example). Nightmare missions only show up on a given planet after you've cleared every single one of the planet's missions. Even after 100 hours I've still not completed many missions, so every now and then I'll knock some out if none of my friends are available and there's not much Void activity in chat. I don't give even half a fuck about the random Capture mission I'm doing on Mars -- I'm only doing it because I initially skipped it when I first made my way through the planet. I'm only here to unlock Nightmare Mode on Mars! The faster I can get through the mission, the faster I can get back to actually progressing.
Hopefully that makes sense. =P
