Is it a bad idea to take a commuter bike for a 50 - 100km ride? I'm a reasonably new rider so excuse my ignorance, but I'm thinking of taking my commuter bike for a 70km ride some time in the near future.

I have recently been on several 30km rides without issue and recently had my bike serviced and adjusted.

The majority of the ride will be in light traffic and there is a shoulder where cyclists regularly ride, so the main concern is not safety. I also consider myself fit enough so fitness should also not be an issue.

So what are the negative aspects of doing this?
Reliability
True commuter bikes are built for day-in and day-out reliability. As far as breaking down, you have little to worry about as a commuter bike is designed to take massive amounts of abuse and neglect and still function. While you could have a part failure like any other bike, these are uncommon and the most frequent (the flat tire) is easily fixed. On a group ride, someone will likely have a repair kit in their bag, or if you are more proactive you can carry one yourself. 

Comfort
While commuter bikes are designed with comfort in mind, they do not have long distance comfort in mind. In particular, a commuter/comfort bike with a wider saddle may prove to cause more chafing on a long ride like this than a narrower and seemingly less comfortable road saddle. Also, depending on your handlebar setup, there may only be one legitimate hand position - which means your body is, by default, forced into one position for the ride. In both cases if after your 30km ride you did not have issues, than whatever issues arise on this longer ride will more than likely be tolerable. While it may not be ideal, you are not going to get any serious injuries due to the bike setup. 

Speed (energy expenditure)
Compared to road bikes, commuter bikes can be slow and heavy. You will spend more energy going the same distance, but if you have the fitness and patience to do so then it's a non-issue, unless you are riding in a group with fit riders on faster bikes where you may hold the group back. Additionally in a strong wind you will have a harder time in an upright position then on a road bike with drop bars, where you can tuck forward into the wind.

While there are better bikes built for longer distance traveling, there is no reason a fit person couldn't finish a 50-100k bike ride on a commuter. Since you have completed 30k I don't think 50k will pose a major issue. If you step up the distance incrementally you will know where your physical limit is. Enjoy your ride!