Its more efficient, also if you can't get the 2nd 250 before you lose friendship, then you get less turns of friendship out of it overall.
Also I would rather know I can afford all 500 sometimes without splitting it up accidentally beforehand. For example, if I spend the first 250 but then NEED to buy a unit or building or RA with the 2nd 250, I will waste a lot of that 250 on influence in the 0-30 range because I can never follow it up. I would rather have tried waiting for the 500, getting to 400 something and then being forced to spend the "2nd" 250 so I never waste the "1st" 250.