Cycle commuting can be an effective way to train, but only if it's a quality workout that progresses over time.
If you live in a city and do the same ride every day (e.g. 45 mins) interrupted by frequent stops for intersections, you'll soon adapt to that training stimulus and stop improving.
Whereas if you have clearer roads and more route options, you could simply follow the training plan workouts on your way to or from work. It's better if you can do the full session in one ride, rather than splitting it in two. Although two rides are better than no rides.
If you decide to cycle to work in addition to your training plan workouts, just be careful that you don't get fatigued from commuting and then feel tired for the key workouts in your training plan. Not all training is beneficial.
In conclusion, I'm all for being time-efficient but I'm not a massive fan of cycling to work as part of your triathlon training. However, in some situations it can work well.