Actually, I completely disagree. As i stated above, I have had half of my customers request a more solid part at increased cost without issue. However, calculating the pricing for the extra filament takes a bit of time and it would be easier if it was built-in.
If the feature was implemented appropriately such as with an image of what the infill would look like and a description of what the best use of the infill percentage would be (Such as 20% - Great for models you need to hold touch and feel but not appropriate for working prototypes. 50% - ok for working prototypes that are under manual forces, likely not strong enough for serious torque or impact, 100% - as close to injection molded parts as a 3D printer can achieve, etc…) This would be a benefit to both hubs and customers.
To overcome the issue you cite above we should make 20% the minimum at the current base price. just offer the increased fill as a premium option.