Earl and Pearl will have self-confidence and individuality issues when they get older. I've done a few posts like this on the baby board of what are parents thinking when they name their twins or triplets, like your Jack and Jill example (and I have met twins with those names). I truly think parents think it's a game when it comes to naming twins. I believe twins should have separate names (including non-matching initials). If parents feel a need to give similar names, do it with the middle names.
When it comes to couples with similar names, like Oliver and Olive, or James and Jamie (a couple know). It's probably very cute for the honeymoon and early newlywed years, and a conversation starter for gatherings, but what else? I often wonder if people look for partners with similar names!