What is the consensus on adding rowing into a training plan???
Some people do, some people don't - it has advantages which may help with triathlon e.g. upper body strength and core
It's clearly a good "full body" cardio workout and, although muscles activated are going to be slightly different to your swim,bike,run training its likely to provide training benefits. I would question why you feel the need to include what is in essence a very generalist session into your plans?
1. For swimming? I suggest for nearly all triathletes pool time and technique will have more of a positive impact and you can always add resistance in the pool or add 20 minutes of targeted upper body strength if desired.
2. For length strength? This should be covered by your strength training.
3. Cardio? If you aren't getting enough cardio training from your bike and run training you aren't doing it right. Whilst you might be limited in how much you can run (injuries etc) if further cardio is required I personally would spend that extra time on a turbo/wattbike/on the road and build fitness targeted for triathlons (remember if you improve your bike strength your run will naturally improve as you have more in the tank).
Long story short, personally I am not a fan, but that's my two cents worth (and my two cents is negatively impacted by spending 3 years rowing when at university)
Fine if there is nothing else available which is most unlikely
Fine as out of season, general preparation training.
Cannot see any cross-over of muscle actions into triathlon so it fails the specificity test of 'race like' training