I love these types of discussion where we can express our opinions without peoples ego's getting in the way!
Experiment: Have your partner come in and thrust as fast as they can and immediately thrust again - see if you as nage can grab their wrist.
I am in total agreement! After we learned the standard/basic.. waza of the technique we (me and the higher ranking aikidoka) like to experiment with the technique via different attacks. I'll do the multiple tsuki thrusts in a row (next time at the dojo) but I don't think I'll get past the first one. The way we train is we always retract the fist/knife from a tsuki attack. I pity the fool who leaves their arm extended after the attack. As my sensei says "those are the guys you hope you fight (assuming you had to get into said fight)" . We also get off-line (irimi-tenkan) pretty darn fast too. It gets interesting when we start attacking each other with out a "set" attack.
One note though: I have learned to not "grab" the wrist on the initial attack. On techniques that usually require an initial grab, I tend to make my hand into a "C" shape so my hand will slide along the forearm but stop once it reaches the wrist. This way I can still react without having to worry about maintaining my grip or having to be extremely precise in my "grab".