Well, Stygian Star's currently at 25 and holding (waitin for a few friends to level their Khelds to similar levels - go team calimari! 
) but I've played all the way there - partly because of the way I learn (by faceplanting) - ie I prefer to DO something to learn it, which makes me a better team-mate, and partly because the only 100% new stuff with a squid is the kheld-only missions and story arcs, and since they start at level 1, PLing wouldnt do me much goos now would it? 
And btw Tech, you actually finished that TF yet? I still want that team at somepoint you know! - assuming that a 25 isn't too low for your rapidly levelling scrapper, that is 
Agree that I really can't see the point in PLing a Kheldian. True my PB has only just got to 14, but it has only taken a few days and it's the most fun I've had through the low levels, given travel power from 1 and a decent attack chain from 6 due to Nova form.
The Kheldian story arcs are a great change from the regular one too, and my first contact is the only one I've needed to use so far (although have finished all his missions now).
However am looking forwards to getting to 20 for the second form. PL plz? 
All very nice points but i just think the most obvious one hasnt been said.
The only reason u might get more "can i get a team plz" from low level Khelds when you'r a high level is simply becuase they can get there.
Both WS and PB can fly by level 6, hence can pretty safely get anywhere in the entire game at pretty much the same pace as a level 50 with flight.
You wouldnt get as many requests from a blaster or Scrapper cos they'd have to go through the Death Race just to get there.
I've just got my WS to 20 and loving every second, i finished my Shadowstar story arc by 16, then was told i had to get to 20 for the next arc, which was fine, i just went off and did Synapse, which whipped me up to 20 in no time. Yum.
Now that i have 3 forms im really starting to formulate a game plan and im sure that as new powers come in it'll alter and change every 2 levels. So yeah i completely agree that Khelds seem to be the AT with the largest learning curve.
Shnyet AWAY
To answer one of the first questions :
IMO, there aren't many PB low-level because they just go through the 20 first levels at an amazing pace. Especially if you go for the nova form.
And you don't need to PL for that.
I also prefer to go solo on the first levels with my Kheldians (three of them now, two PB and one WS), simply because otherwise I spent most of my time waiting for others to arrive at the mission door (ok, the WS is a little best regarding this cos it gets recall friend at lvl 10). And you tend to upset people cos they get nothing to do because you blast everything before they can even reach the mobs. Low-level Kheldians are god-like beings. I'm probably not alone in this case, so don't expect to see many of them before lvl 10-14.
About PLing a Kheldian, especially from low-level : I find this simply stupid, 'cos they get specific SAs. If you're interested in the game, you don't want to zap them. Now, I've seen people who don't care much. But these are typically those lvl 50 who ask you some silly questions which makes you believe the account is being played by someone else.
Well I loved playing my PB from 1-50, the last thing I would want to do is powerlevel it. Around about the time I started mine there was another one starting, within a week they were in the high 30's, so I asked them 'I take it your powerlevelling?', their response 'yes, i've done all the missions through once with my main AT so little point doing it all again' my response to that 'you do realise Kheldians have their own story arc/missions from level 1-50 don't you?'
I see little point powerlevelling as once you hit 50 there is nothing to do anyway so why rush it? And secondly of all the AT's Kheldians are the only ones with their own arc of missions all the way from 1-50, so why skip that?
I can't say i've seen Kheldians asking for powerlevelling more than any other AT though, a simple look at the Defiant level 50's chart shows that they are still low in number. If they were powerlevelling all the time there would be a lot more.
There are good and bad players, same goes for Kheldian players.
As for what's being good about a high level Kheldian, awesome power. Used properly PB's and Warshades are a match for any other AT out there.