Does it really matter how the dwarf would get a 36 strength? Even though a couple of dragon discple levels would boost it way beyond 36. The question remains the same for any high strength character. Of course, weight in itself has nothing to do with the effectiveness of the towershield as a weapon. Balance and ease of use is probably a better way to say why it cannot be used.
In any event, I have not seen a huge increase in the use of towershields in 3.5. The fighters who have the skill and strength to carry such a dead weight tend to fight with two-handed weapons. The spellcasting classes tend to come in medium or heavy load by just carrying the thing.
