True 'dat. Depends what you mean by "BBQ." If you mean cooking low and slow and mostly indirect (smoker or open-pit), it's pretty difficult to get the meat's surface temperature hot enough to actually scorch the sugar. The actual scorch point of sugar is 350F. That's not an oven temperature or air temperature (convection) but a contact or immersion temperature. So, I'd have to disagree with the reasoning behind this. While you sometimes see enough heat to do this in a "Santa Maria" style open-pit barbecue, it's uncommon anywhere else. In fact, it's almost always the sign of a poorly managed pit.
Anyway, when it comes to 'cuing tri-tip, whether direct or indirect, sugar is not a good rub component.
Remarkably untrue. You get maximum penetration, about `1/4" of dry rub, within 30 minutes. With a few exceptions, long, "dry marinades" are pointless.
This is the statement that brought me back into this thread -- because it's so wrong. Chef, remember what you said at the top of this post, how sugar burns? That's exactly why you don't want much sugar, if any, on meat that will be grilled over a direct heat. It will not form a crust ("bark" in barbecue language), but will char quickly, than burn. Burnt sugar is never desirable because it's bitter and smells bad.
If you like, you can a sweet sauce at the last minute and get a little char on it, than pull. Alternatively, you can sear, mop with a sweetish sauce or glaze, then finish cooking indirect.
The reason the OP's rub is "melting" is not because of the sugar, but because he's essentially braising the meat. The meat "sweats" during the cook, and the moisture dissolves and washes the rub off.
This needs some 'splainin'. Chef, forgive me, but I don't think you "understand what types of sugars you use and how they react to heat," either. Granlulated sugar is crystallized, refined cane juice, crushed to the appropriate texture. Brown sugar is granulated sugar plus a little molasses -- light brown has less molasses, dark brown has more. Powdered sugar is crushed granulated sugar plus a little cornstarch to keep it powdery. Since all three are pretty much granulated sugar," how do you think they differ in their response to heat? Hint: They don't -- at least not by much.
As a general rule, it's a good idea to reserve sugary rubs for slow cooking pork. For all other meats and techniques, if sweetness is desired, it may be brought later. This is true for barbecuing (indirect or open-pit), high-heat indirect (as in a Weber Kettle), and trebly true for grilling.
Sorry to come down on you so hard, but it's important to keep these things straight.
Hope this helps,