Why the constant pounding in IM training? As opposed to fewer, longer days?

That's an easier way to get injured and a harder way to build fitness.  Consistency is the key (NOT the length of your longest workout).  And your fitness is the sum total of all your training.

You should be tired & fatigued when building for your IM.  But if it is "too much", then you need to check if the plan you are using makes sense for you and adjust accordingly.

X2. Smaller amounts of constant, consistant pounding gives your body a better chance to adapt and become stronger. If most of your workouts are mega-workouts, then the odds of your body getting beat down to the point that they have trouble recovering is higher, then injury comes a callin. This isn't to say that I don't believe in the mega workout, it's just that your have to save your gun powder for the right times.

^^^What JK and TJ said.

As a f'rinstance, I found it harder to avoid running injuries when I transitioned to tri training from full-time running only and was still doing the same long weekend runs as before (esp. when ramping up for a marathon while still trying to maintain a respectable swim and bike regimen).  The simple reality was that I wasn't supporting those long weekend runs with enough running the rest of the week and the long runs had the same effect people caution about if you try to up your total mileage per week too fast.  Consistency is really important.

We all know the stereotype of the "weekend warrior" who gets up off the couch for a game of football or whatever with his buddies on Sunday and hates life the next day...there's a reality to that stereotype.

An important part of IM training is preparing your body to better deal with fatigue and increase the rate you are able to continue while fatigued. The consistency is key to building this. But I think being bruised, beat up and exhausted sounds like overkill, I'm tired but I'm not broken down. You may want to adjust your plan a bit to avoid burning out.

I am a first time IM wannabe going for Florida and I have recently had to move to a schedule like you are talking about.  There has been some career stuff come up that is not allowing me to do quite as packed a training schedule as I had planned.  So I have increased the distance on the days that I am training.  Now in a perfect world I can see where getting you total mileage through a bunch of short workouts and one long workout would be safer and preferable.  However, I believe that a strong level of fitness can be obtained with slightly less total workouts but with longer distance.  I hate to admit it but my body is feeling better since I made the switch.  I can tell I am not getting quite the level of fitness I had previously but I have also backed off the running which may account for that.  I have sacraficed the run (my favorite event) in leiu of focusing on the swim and bike.  I just want to make sure I don't spend tons of time training for the run of my life only to miss the swim cutoff.

Good luck with whatever you choose to do.  It sounds like you have a good enough base that you will be fine either way. 

First, I agree with the previous posts. I had similar questions on how shorter workouts would benefit me, but it is the cumulative effect of all those workouts. There's enough stress on the body with the weekend long rides/runs which seem to require lots of recovery - even if you don't feel it initially, it wears you down eventually.

Secondly, I'm surprised to hear you're doing so many all-3-sports-in-one-day workouts. My plan (Don Fink's) NEVER calls for that - except for the couple races thrown in there. It may be 2 workouts in one day, or a bike/run brick, but never all 3.

Third, I too think the workouts become relentless - especially over the last month or so. A necessary evil. This taper is so mentally refreshing...and I haven't really cut back that much from a physical point of view.