Yes, I agree that reaction would probably be far too slow to be useful. I hate to say it would never happen though!
As for nucleophilicity versus basicity, actual quantifications of these effects aren't as important as the relative properties of different molecules. Take my azide/triethylamine example: you can pretty easily rationilize why one would be a good base but a poor nucleophile based on sterics. Most cases are like this, so it's not all that hard. The thing you need to remember is that the two effects are different.
I think of them this way: basicity (pKa) is an equillibrium measurement. You would put your molecule in a solvent, let it sit for a while, and measure how much is dissociated.
Nucleophilicity, on the other hand, is a kinetic question. In two nucleophilic displacement reactions the products are essentially the same, so they are likely similar endpoints, thermodynamically speaking. The difference we might expect to observe is which nucleophile reacts faster. A really great nucleophile could react in seconds, while a poor nucleophile might take ages.