When the nodes, especially Rahu, join Mercury it is actually one of the better placements.
Rahu is said to be very strong in Gemini and Virgo. This is because Rahu is where we are trying to understand things intellectually, trying to be flexible and figure things out which are also Mercury qualities. When you get Rahu and Mercury, it can bring some intelligence to the random quality of the nodes.
Rahu / Ketu and Mercury
The nodes, especially Rahu, can be destructive when we take action and act based on immaturity. We are here to take action but the actions of Rahu can be very difficult. This is why, for instance, when you get Rahu /Mars it can be destructive because Mars takes impulsive actions.
Mercury is not really a planet of action, it is a planet of skill and intelligence. Often, with Rahu/Mercury, we try to figure things out before we actually do something about it. This is just the nature of Mercury but the problem is we do not take a lot of action. We just think about things and that is actually good with Rahu because Rahu is where we are trying to figure things out.
Rahu is very open, immature and trying to figure things out and understand how they work that is why we act with Rahu. We act with Rahu not because we are trying to do stupid things but because we are trying to understand what that thing is. And often, the consequences just happen to be the stupid things that we do. Rahu/Mercury can be quite uplifting in bringing about positive change for Rahu energy.
With Ketu, it is a little bit different. Ketu is where we are already kind of serving up something. That is not so good with Mercury because Mercury is where we are trying to figure it out while Ketu is where we think we got it all figured out.