Re: whats wrong with cokers?
The only real answer to the coker vs. giraffe question is “Both, but Coker first…” 
Giraffe is gratuitous uni, but Coker is essential transportation.
To your other question, I’m sure others will jump in with more detailed responses, but here are my thoughts as someone that has made (or at least paid for) many modifications.
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For basic distance riding, there is nothing “wrong” with a stock Coker. I think most people with stock Cokers are perfectly happy, as was I when I had one. People are putting thousands of miles on them with no problems. BUT…there are two issues with the stock Coker that tend to drive most of the modifications.
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The first and most common modification is a seat and handle upgrade. Look at what the stock one comes with…a basic hard seat and no handle. Not very compatable with distance riding and long saddle time. Some type of airseat is a huge improvement here, as is at least a basic handle addition like the GB4, which not only gives you a way to get some of your weight off the seat occassionally, it also gives you leverage for climbs or descents, and a place to mount your computer.
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The second and less common (but more expensive) set of modifications involve upgrades to the rim, to the hub and rim, or to the hub, rim, and frame. The stock Coker rim and narrow hub, combined with the large wheel diameter, make it weak overall. Maybe Brian McK will post a link to his famous video illustrating this weakness.
In addition to overall weakness, the stock rim is flexy, which makes smooth braking a challenge. Not everyone wants or needs a brake, so not everyone cares about this. My knees are a horrorshow, and I need a brake, so the upgrade was key for me. The combination of the custom Airfoil rims and one of the new wide hubs makes the wheel much stiffer and stronger, which helps both for braking, and for rough terrain or small drops which could taco a stock rim.
As for frame upgrades, I think a lot of them are driven by the combination of the extra-wide hub, which doesn’t fit the stock Coker frame. Plus the nice custom frames like the GB4 or Hunter are stiffer and more durable. Plus they’re just eye candy…very nice to look at and very cool to own. http://gallery.unicyclist.com/GB4-36-custom/IMG_0632
I remember a post from Nathan (which I can’t find now) where he talked about all the different iterations of coker setup he had. I think his summary was that the wide hub and airfoil rim upgrade had the single biggest effect on the performance and stability of the distance machine. I can’t say from my own experience, as I did my upgrade all at once, going from a stock coker with airseat to a GB4 36 with U-Turn’s “Strongest Coker Wheel” and other bell$ and whi$tle$.
I can’t say for sure whether having the nicer setup has made the difference, but my cokering ability has improved significantly since getting the custom machine. That might just be because I’ve now put twice as many miles on it as I had on my original stock setup.
So to net that out, I’d say unless you know you’re committed to putting serious distance on the thing, and will be riding on rough terrain, and will be wanting a brake, a stock Coker with an airseat and handle upgrade should be quite sufficient.