I'm curious what people instinctively do and whether anyone has data on it. When I go to About Us, for example, I click on it rather than on a sub-lelvel, which usually has "Meet the Team," etc.
Our analytics and heatmaps show that users go straight for the "main category" link more than I would expect, but then, I designed the dropdowns and interactions, so my expectations are a little biased.
It's not exactly hard data, but I've generally seen that of all the clicks garnered within a menu section (a main category link and sub category links in the related dropdown) the main link is responsible for 20-30%, with the rest of the clicks distributed among the other links.
That said, there are several factors that might affect this:
Verb-based labels see more direct clicks on the main links that more passive descriptions. The top level link that's clicked directly most often is "Contact Us", much more than labels like "Company" and "Solutions". (though this could also be a result of the users expectations being set by convention)
This is speculation, but I'd bet that in an A/B test between the labels "Products" and "View Products", the latter would see more direct clicks.
The speed and type of interaction on your menu will play a factor. If your dropdown doesn't snap into place quickly, or the interaction is clunky, non-conventional, or generally dissatisfying, your top level links will probably end up with more clicks.
We've run into issues with mobile devices where we hadn't built in touch-to-hover functionality. So anywhere an interaction was on hover, like the main menu dropdowns, the user only had the option of the top level link.
What to do?
I think that unless you're going to build the menu such that the function of the top level links is merely to display the dropdown, then your focus should be on those main category landing pages, and on making sure they're well designed and fulfill their function of moving traffic to deeper pages.