When leaning out, I use zero to one shake per day (post workout). When bulking, I consume 3 shakes per day (to keep the tank full). I've noticed a big difference in how I lean out from when I used to consume 3 shakes per day when leaning out. There is a lot of info on here relating to this topic. Specifically search for posts by Bobo as he has some valuable insight on this.
CDB put it in a nutshell. When you consume a liquid, it will enter small intestine in 15 - 30 minutes. This is a short term fuel supply. If you were to try and bulk using nothing but liquid supplements, there is no way that you could sustain the nutrients needed to put on muscle mass. Your body would spike insulin and then crash (and would encourage fat storage) and would get a large hit of aminos and crash....then comes catabolism. You wouldn't have a sustained glycogen level nor would it (IMO) process aminos/fats in a manor needed to build muscle. You also would start shi tting bullets.
The reason that a post workout shake is so important is to replentish your body of amino and glycogen deficiencies caused by intense training. The whole idea is to get something in your system which digests/absorbs quickly in order to stave off catabolism. If you read around the board, you will find that it is also recommended to eat a whole food meal about 30-60 minutes after consuming this shake.
With respect to adding fiber to slow absorption, you need to make sure that you add the correct fiber or you will get the opposite effect of what you are looking for. Really, it primarily slows/increases digestion.
soluble fiber = decreasing digestion (absorption with respect to glucose)
insoluble fiber = increasing digestion (absorption with respect to glucose)
http://anabolicminds.com/forum/weight-loss/25399-why-natty-pb-before-bed-2.html#post506708
http://anabolicminds.com/forum/weight-loss/28410-bobo-help-us-rice-oligodextrini.html?highlight=soluble+fiber