It's a good point about grappling and street defense in general. It's very easy for someone to have a knife, utility razor-blade, or even small pistol concealed and available to be pulled out quickly. If someone has such a weapon, wrestling or throwing them might not be such a good idea.
However, in the context of my Aikido practice, speculating about what my partner could do with the addition of a hypothetical weapon is a bit too hypothetical to be of any importance. Most Aikido training is like a science experiment: zillions of possible variables are taken to be constant in order to focus on dealing with a specific situation/problem/principle. If you get too caught up in what-ifs, training devolves into a bunch of standing around and talking.