Is there a functional difference between bricks w/ and w/o bottom tubes?
Parts with and without bottom tubes are nearly always interchangeable in normal building scenarios.

However, there is definitely a functional difference. Not having bottom tubes generally does the following:


Reduces clutch power. There's one less surface for the stud to produce friction against.
Allows for lots of different connection points along the bottom edge rather than just at both ends. This can be useful in places where you aren't always working in whole numbers of modules. For example, here's one way that you could build a wall at 45 degrees to what you would normally build by exploiting 1x2 bricks without bottom tubes: