Chain Merging is for gaining more training points in the least amount if days/exp.
It has little to do with leveling (other than preventing it prematurely), and only indirectly helps aptitude. (i.e. you can buy potions that convert 1000 training points into 1 aptitude) Training points are, as far as we know, only used to convert into either TP or Apt (500 to 100tp or 1000 to 1 apt), so 2500 training points will increase/learn a passive.
Personally, I find Golden Eggs pretty easy to harvest myself while I'm watching my nightly TV or w/e. I find it amusing that people are willing to pay even 20k for them. And assuming you're merging in runic pets, each one gives 100 training points, so you only need 40 of 'em for 4 points of aptitude. ('course, this is presumably also why runics tend to be more expensive, but if you aren't able to chain merge, it's *real* slow gathering training points with non-runics).
But yeah, I have a pet for both sides of my character, so until they add in ways to get additional breeding slot tickets, I can't really take advantage of chain merging either...
('Course, I haven't hatched either of the good eggs I've obtained, I'm waiting until I *can* do chain merging until I do... eventual plan is to try to get 'em to 90 gaining as much training points as possible and keeping them as low level as possible, (using chain merging to optimize the training point learning) then apt potion my way to 100.) Sadly, I now realize I wasted a lot of good eggs on what it now appears will be throwaway pets. (I thought it would be longer until I got holy versions)
But yeah, I find it pretty funny how absurdly high some people are asking for their 80+ apt eggs... almost as funny as the fact that some people are actually buying them... I mean really, 1.5 mill for a lvl 25 85apt egg or w/e? It's growth rate is 1/1/1/1/1, so not useful in itself, and at lvl 25 the apt is pretty much too late to help another egg out anyways.... and similar other mindboggling sales (or eggs FOR sale). Heck, on my server there's a lvl 38 or something egg with like 95 something apt that was initially priced at like 5 million... now it's come down to 3-4? (3 bid, 4 buyout?) Dude. Your eggs good for one thing. Training points and leveling. Same as any other random runic. At lvl 38 it is *way* too late for the apt on the egg to make ANY difference, and the training points will only be worthwhile for a lvl 38+ egg, so if anything it's worth LESS than most other garbage runics. Whatever, if he's smart he'd price it at 500k to 1 mill, probably still somebody ignorant enough to buy it at that price, whereas if he waits much longer nobody will be.
'Course, I'm obviously in no hurry to raise my eggs, so am perfectly willing to sit and wait for those ~20k high apt eggs to come up on the market and/or wait until all the early adopters with more money than sense have raised their eggs (or run out of money dumping high apt eggs into pets they end up tossing) and prices come down to meet my demand

.