I am not qualified to comment on this subject as it pertains to racing, but I have done a lot of this kind of paddling over the past few years.
Though I have the balance of a one-legged fat arsed Hippo running on a log, last season I was averaging 24 to 37km's(15 to 23 miles) an outing, 3 to 5 times a week, with one week of 157km in a 5 day period.
For the most part, above all else, long and slow eats up a ton of calories; about 2000 to 3000 a trip give or take.
Let's face it...If your going to have the courage to walk around dressed in Spandex shorts, calorie burning (as well as a strategically placed sock)can be very important.
Because of the high calorie burn,if done in excess, the protein needs of long and slow can end up having your body eat up a good amount of muscle; thereby sapping your overall strength and in the end, reducing your shorter distance sprint speeds. If overdone on a regular/daily basis, it can even cause a slight weakening and atrophy in the legs muscles. So you have to make sure that you devote a few days a week to strengthening them also.
From my personal experience that's the bad that comes with overdoing long and slow.
As for the good...
For me personally, I have found that long and slow is very significant when it comes to improving your "grunt conditions" conditioning and performance; such as situations where you come up against current and/or, wind.
It can make aggravating, drudging paddles, against inlet currents and tides, a non-issue. Long and slow can also make upwind paddles your strength, when for many others it is a weakness.
When you hit an 8km stretch of water that has a 3.5km current running against you, combined with a 24-32km wind blowing in your face, long and slow can give you the ability to methodically and effortlessly, cut through the conditions like a hot knife through buttah.
Never breaking cadence... Never breaking stroke...
What I like to describe as a, beat your chest, manly-man, grunt, growl and snarl paddling experience.
( Start "The Gladiator" theme music here...)
Just you with the wind and spray in your face... Digging your heels into the foot board. Your back and shoulder muscles firing to a full pump, as you pull your cement laden paddle through the water without even giving it a second thought...
Freakin' RIGHT paddle stroke...Freakin' LEFT paddle stroke, (Insert spit over the gunwale and manly grunt here)
Freakin Right paddle stroke...Freakin LEFT paddle stroke, (Insert a second spit over the gunwale and manly grunt here)
THAT'S the benefit of long and slow !
Not to mention the fact that it can make you so tired, when you get home you won't even hear your wife yelling at you for the entire day.
Or is that just me?