1. When I was playing it was my opinion that Rifts hit too often and I posted in their forums saying so. Yes the number of Rifts spawned is gauged by the number of players on a server. Unfortunately that doesn't take into account players get bored of doing one thing. Once players get past the novelty of them it can become a nuisance to have to continually do them all the time. My feelings have been I am trying to finish my quest, I just got done with the last Rift invasion, which took AN HOUR, I've got limited play time, I'm not going to another Rift unless I have to, I'll let someone else do it. I like having to band together to protect npc's occasionally but I don't like having to do it all the time so why go if I don't have to. I think the frequency of Rifts needs to be turned down slightly then you will see players doing them more. Rifts aren't bad but Trion may be padding the game length just a bit with them.
2. It's a question of contrast. For example I've heard Spielberg talk about making Jaws. He liked the scare he got out of a test audience so he went back and edited in more scares. At the next testing he noticed the audience was less scared with more shock moments in the movie. In short it's about pacing. Do Rifts to often and they feel less special and less imperative that you do them.
3. As far as the gear goes because the gear you can buy is based on the area you farm the sourceshard thingy in it may be best to skip over buying it and keep the Planarite for the next area. At 30 I could have skipped farming for the Gloamwood gear and actually bought and used the next area's gear instead getting more bang for my buck.
4. I expect there are a lot more people in the beginning area and there are more newbies less bored with invasions. Wait for the more casual players to level up and out of newbie land and you'll have rift help. Not everyone has time to power level as quick as possible.