Why the first 50 addresses with non-default port number are considered same as invalid?
To be honest, I have no idea. Just like I have no idea about a purpose of many things this s/w does, also spotted BTW.
But my personal experience advises me to not ask these kind of questions, because they don't like it.
It's a feature, not an issue - that's all you need to know.
The bitcoin elite is an actual elite, so if they made an effort to add a line of code - who are we to question it?
IIRC, You asked this previously and it was explained to you. Bitcoin preferentially connects to the default port if it is able to in order to reduce the problem of a malicious party announcing other peoples addresses maliciously and creating connection DDOS attacks against random ports. Its important that Bitcoin not act as a network nuisance and get itself banned from networks, so it doesn't use non-default ports unless the default ports aren't working. I can't see into Satoshi's mind, but the rational seems clear enough for me.
Adding additional nodes on a single network is not of tremendous value to the network, though if you'd like to do so for local scaling reasons (instead of just increasing the accepted connections on a single node) you can simply load balance at the front end, iptables will do this happily, as will many commercial routers and firewalls. I've had multiple inbound nodes on a single IP this way without issue. If there was evidence of a problem that could be solved by having more nodes on distinct ports treating a small range of high ports as equally preferred, which would also avoid the nuisance/DDOS risk. ... but I haven't seen a reason to even propose a BIP for it. If you have one, I'm all ears.
[Sorry for continuing the OT here, but I'm somewhat concerned that piotr_n's misinformation— if unanswered— may adversely impact my personal reputation as well as the stability of the bitcoin ecosystem, e.g. if people who don't know better see his unanswered allegations that core developers never explain anything they may believe it]
Everything is just fine. 
