ThirdWizard said:
Leaving your other thoughts aside, retroactive skillpoint gain would often necessarily involve gaining skills you didn't posess already. For exmaple, if, you max out your current skills, spilling over into others would happen.
What about oddities like a cleric 3/fighter 8? He hasn't been a cleric for nine levels, but suddenly he gains ranks in cleric skills? Where did that come from exactly? What's the rationale behind that? Especially if he used all his cleric levels to increase jump, climb, and swim, which are now maxed out in ranks, and he takes Knowlege (religion) which he didn't have previously!
Where does it come from at all? I daresay that most levels for anybody with a decent int involve new sklls. Class skills vs non-class skills is a red herring. At each new level I can gain new skills regardless of my class. I just pay more for some than others. That caveat notwithstanding, where does your cleric get his skills for his first fighter level? Did they just pop into his fron? If you've got say a 3rd level character who had enough skill points to max out the skills he did take and no more, then he gets his next two levels and maxes out again. Repeats this for fourth but now due to an INT increase has an "extra" skill point. So now he picks up .... wait for it .... a NEW skill!
Any fighter can take ranks in Knowledge (Religion). Sure they pay double for it, but they can do it. WHere is the problem? Where is the disconnect? What is different in this case from a normal fighter picking up a new skill such as KNowledge (Religion)? There is no difference.
Now, perhaps you mean "backdating" skills? What so difficult in that other than the greuling paperwork? I'm reasonably sure if you gave it serious thought you'd find yourself in similar situations, and it made sense. Not RP, but RL. Getting smarter can certainly mean you are better in skills you haven't used for a long time. Like riding a bike, as the phrase utters.
Here is how I do it. You get the skillpoints, but they are achieved in the class you levelled in. Unless you've got another house rule that says any skills you hvae as class skills for one class are counted as class skills for any class you level in, you can get non-class skills, but just like any other level you pay more for them. Sure, since one of your classes have them as class skills they can still be kept up above the non-class level, but it will cost you double to do so.
Just like with con improvements, you are
NOW smarter/hardier. You are not smarter/hardier
back then. Backdate is not the same as retroactively gaining them. The difference is retroactively
gaining versus retroactively
applying.
An intelligence change is global. Skill improvements by level are local. Any change to the global affects the local. You can also think of it as "now you have more room for stuff"; as in "your brain gets bigger". Heh, thinking of it that way you'd have to have INT be retroactive in skill points .. otherwise the bigger brain has a lot of emptiness.
How or why it goes one way or the other is, as one postrer said, flavor and nothing more. I was primarily responding to the notion that you don't just pick up new skills suddenly in real life. Indeed it is quite the opposite as attentive parents and teachers of young children will tell you.
Personally, I'd prefer if all clases got the same skill point base, and a player could choose an additional class skill for each INT bonus. Yes they'd get a new "slot" when they had a permanent increase.

Heck maybe each class could have a class skill allotment and you choose enough class skills to fill that allottment (plus bonus slots). The more "skill oriented" ones would just get more slots.
That said, as someone who has been a fighter (martial artist, advanced recon soldier) and a programmer and sysadmin; no, one does not have any more affinity to what we'd call skill points. Soldiers actually need to have a *lot* of skills.
Lastly, sometimes picking up a new skill happens by "critical mass". You aren't trying to learn it but by being around it's use or hearing it being talked about, etc. eventually, it just "clicks". Yet it appears to be a "sudden" event. I've seen this happen a *lot* in the tech industry. Much like how "overnight success" never happens overnight.
Cheers