It's relative, in theory you could pump intelligence to the level of a Wizard in the DnD sense, certainly if you so chose. As you said your combat would suffer, but you could. However would you be intelligent in the same way as the Wizard? Without the comparable Wizard levels to said Wizard you wouldn't have any of their Wizard oriented knowledge or specialization that they might add to a conversation. And, really, why would you?
That said the same happens on the other side of things, all things being of equal intelligence, and you're a Fighter, a Wizard wouldn't necessarily have your knowledge of physical combat tactics . . . though one could argue, being you've upped your intelligence, and neglected your combat prowess, that you might not know some combat oriented stuff that another Fighter might know despite being less intelligent. This is where I come into the idea that the Warrior with lower intelligence would be able to accomplish his goals as readily as the Wizard, he'd just do so through different means. No less capable, just capable of different things by the chocies he made. Statistical choices and Class specialization choices aren't necessarily linked. I'm not sure that you should be able to do, 'what that guy can' just because you're both smart, as your class choices represent different life choices and knowledge bases.
that's fine. the warrior doesn't have to do it the same was as the mage. The class should be a factor too. I'm just saying in general, making an intelligent or persuasive warrior should be just as viable an option as an intelligent or persuasive mage. They could use the intelligence differently. I don't want my warrior to start reciting spells and I don't want my mage to threaten to club a guy with his staff. Both classes would have the choice of being great at their combat skills but bad socially, or they could give up some combat skills to improve their social skills. Most RPG's like this basically push you into being a magic user if you want to have good social skills.