There are a lot of reasons to use maltodextrin beyond things already mentioned here...
Maltodextrin is added as a clean source of carbohydrate, not as a sweetener. If you used dextrose, you'd be making your drink horribly sweet. (Want to remember the sweetness of dextrose? It's what Pixie Stix and Smarties are made of.) Since substituting a sweet carb is not a good flavor alternative, you'd have to go to a starch. And there's already a lot of starch and complex carbs from other ingredients.
Maltodextrin is a more complex carb than simpler sugars - it's really between sugar and starch. As such, it has some more starch-like qualities in food than sugar-like:
Sugar can make powders clump over time, maltodextrin stays fluffy and mixable, acting as an anti-stick, like cornstarch. Yet maltodextrin dissolves super-easily in water.
Maltodextrin has a smoother mouth-feel than sugars, making foods almost seem like they have more fat in them than they do. Makes the shake more "shake-like" and less like "wet oat flour."
And, contrary to some posts, there are values to having some quickly-absorbed carbs. Assuming you're hungry when you eat, the quickly-absorbed carbs are the first to hit your bloodstream and actually give your body the energy it wants right now. This is why maltodextrin is used in many sports and recovery drinks. Also, getting sugars into your blood contributes to the feeling of satisfaction from eating - but not if ALL you get is sugar. That turns into its own problem. A appropriate balance of protein, carbs, and fats tends to more satisfaction while eating and greater feelings of satiety after.
You don't need to sip your Soylent all day to prevent a sugar rush - although the GI of maltodextrin is quite high, in a mixed meal, the overall GI comes down. Absorption of the maltodextrin is slowed by the protein, the fats, the fiber, even the other complex carbs.
Lastly - on the high GI - there's no reasons foods can't have a higher GI than 100. It's not a 100% of possible scale, it's a scale which was set, at the outset, so that glucose rates a 100. This does not mean that nothing an have a higher GI than 100, just that glucose happens to have a GI of 100. Since glucose is a very high-GI food, not many will have higher. A maltodextrin with a GI of 110 just means 10% higher than glucose.