The reason you would not use a protein/carb supplement as a pre/post formula unless it contains faster oxidizing sugars (see below) such as maltodextrins as the carbohydrate (CHO) – i.e. you want the quickly absorbed CHO available if you want to get all substances into the working muscle area as fast as possible such as before the workout starts and the anabolic window closes post workout, or at least during its widest period and to refill glycogen. The goal for all athletes at the onset of exercise is for the stomach to be as empty and possible and muscle energy full, and following exercise you want immediate delivery.
Also, maltodextrins, like many other glucose polymers/compounds can be used as an agent to make food products texture, taste or uniformity superior and this may be where your friends are getting mixed up.
Lastly, make sure when “experts” claim something, they have peer reviewed research to back it up, not solely internet fodder or magazine marketing.
More info:
Carbohydrate –Maltodextrins
WheySmooth is primarily a fast-acting protein supplement. The carbohydrate content in WheySmooth is strategically designed for 1) minimal calorie contribution allowing adjustments (added foods/fluids into mix) as desired; 2) rapid gastric emptying thus oxidation during pre/post workout use to help maximize MPS post workout; 3) flavor and easy mixing properties.
Maltodextrin
Maltodextrin is a polysaccharide. It is a lightly hydrolyzed starch used as an ingredient in many food products as a thickener and carbohydrate source. Maltodextrin is easily digestible, being absorbed as rapidly as glucose but moderately sweet or sometimes bland making it desirable in food manufacturing.154 Carbohydrates in sports are generally placed in two categories. Those that can be oxidized (used for energy) rapidly (up to ~60 g/hr) and those oxidized slower (up to ~40 g/hr). Maltodextrins like glucose, maltose and sucrose fall in the rapid category. These carbohydrates are digested and absorbed at rapid rates making them readily available to working muscle and, when in small amounts as in WheySmooth, do not slow down amino acid absorption from protein. These qualities, including maltodextrins food mixture compatibility, make them ideal in a product like WheySmooth that’s designed as a low (but flexible) calorie meal protein supplement and/or pre/post workout protein supplement.
From other peer reviewed studies:
There has been general agreement for the last decade that sports drinks need to contain salt (sodium chloride, NaCl) and carbohydrate (sugars) at concentrations of around 20 mM and 6% (6 g per 100 ml) respectively. The researchers also agree that at least some of the carbohydrate needs to be in the form or disaccharides (usually sucrose) or glucose polymers (maltodextrins). In the last year or two Jeukendrup and colleagues have found a way to increase the rate of absorption of carbohydrate including the use of maltodextrin mixtures.
There are several studies that link the increased exogenous carbohydrate oxidation rates observed with multiple transportable carbohydrates to delayed fatigue and improved exercise performance. In one study, subjects ingested 1.5 g/min of glucose:fructose or glucose during 5 h of moderate-intensity exercise, and it was observed that the subjects’ ratings of perceived exertion were lower with the mixture of glucose and fructose than with glucose alone. Cyclists were also better able to maintain their cadence towards the end of 5 h of cycling [33]. Rowlands et al. confirmed these findings and reported reduced fatigue when ingesting a maltodextrin: fructose mix (maltodextrin is a glucose polymer with little sweetness that is very rapidly digested and therefore behaves identical to glucose).
Asker Jeukendrup, et al. A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise. Sports Med. 2014; 44(Suppl 1): 25–33.
Published online 2014 May 3. doi: 10.1007/s40279-014-0148-z