re: Table saw sleds, How 5 cut do you need to be?

I almost always use my sled for 90° cuts.  I followed William Ng's design fairly closely.  I have not checked it in a while to see if it is still square but I have not noticed any problems with non square cuts either.   I make a lot of cuts with my sled that I would not even attempt on my table saw without it.  I have an Incra miter gauge but only use it for cuts other than 90°.  For fairly long pieces, I find the miter gauge cumbersome and for relatively small pieces, unsafe unless I attach an extra long fence to it that extends  past the blade.  A long fence on the miter gauge adds more variables.  It can flex and add error to the cut, for example.  Also, especially with small pieces sitting on the table, the friction with the table adds another thing that can cause pieces to shift.  The fence on the sled also ensures that the cuttoff piece, which would be unsupported with a miter gauge, is pushed through the cut reducing issues with the cut off pivoting at the end of the cut and being damaged or even splinter off at the end of the cut.  

EDIT to add: It seems to me that there may be 2 more constraints to consider which may be special cases or use cases perhaps.    
First, when cross cutting really long pieces, the weight of the unsupported end can make a cross cut difficult because the supported end with tend to pivot upwards.  With a miter gauge, this can be difficult to achieve but with my sled, I can use a clamp on the fence  to help hold down the short end.  
The second thing is eliminating issues that friction between the piece and the table can cause, like the unsupported cut off mentioned above.  A sled eliminates this by pushing both sides of the cut completely past the leading edge of the blade.