I can think of two possible reasons that people would use alliterative (or rhyming) names for their multiples. One is just that they think it's cute and match-y, and they're not necessarily thinking of the kids living their lives with similar names.
The other one is the bond that multiples feel. I'm not a multiple and I don't have any as kids but I've heard various stories about how close they feel and I wonder if some people choose similar names to enhance that bond. I don't think similar names necessarily make them feel that bond any more than with different names, but I wonder if that's in some peoples' thinking.
This reminds me of a family I've mentioned before - they named their 1st daughter with an A name (
Alyssa), 2nd with a B name (
Brielle) and then they had twins - so they both got a C name (
Candace and
Ciara). I think that is as good as reason as any to use the same letter - although I guess they could have given one a C and one a D.