It depends, some people want their kids to have their own bathrooms so they both don't have to deal with it and teach their kids to clean up after themselves. One family bathroom which is the pretty old school normal way, do kids ever clean the bathroom, not too likely.
Overall as its nice to have your own space, its nice to have your own bathroom. Most people don't need huge bedrooms, depending on the size of the house if the bedrooms are tiny and they stick in an ensuite to make it barely bigger than a single bed in the bedroom space, its stupid, if you've got enough room for wardrobes, desk and double bed comfortably, then why not stick in a en-suite. Bathrooms are relatively cheap, an extra stud wall or two, a cheap bathroom suite, tiles, plumbing(done in new build rather than retro fitted and with usually a couple ensuites backing on to each other) the cost is minimal.
Its housing development, you make a house that will appeal to as wide a range of people as possible for the market area so it sells as quickly as possible to free up capital for other projects.
In an area that is 99% families, 4 en suites and one cloakroom downstairs would be very difficult to find, in an area that is 50% familes and 50% professionals, or students, you're more likely to find people renting, often with people they aren't that close to and a en-suite can save many problems.
Personally I think in general these days what you'll find is a master bedroom with ensuite, a family bathroom and at the very least a cloakroom downstairs, if there is plenty of space sometimes you'll stick a shower in downstairs, for when you come in caked in mud, or when your kid breaks his leg and can stay downstairs for a couple months, or your gran stays over on the sofa bed downstairs and doesn't have to deal with stairs, etc, etc. Another thing that is far more american is a jack and jill style ensuite, rather than having two tiny bathrooms on either side of the same wall you have a double sized bathroom and a door from each room.
More bathrooms, particularly adding one downstairs adds to the flexibility of the house, as said in some markets its a waste, in others it vastly increases the interest in a property, which means it sells for more. We had one bathroom till I was... I dunno 13-14, then a loft extension meant another shower and it did dramatically decrease the time it took for everyone to get ready for school/work in the morning, if everyone had their own toilet and shower.... it would have been easier again. If you've got the space I'd choose en-suites vs oversized bedrooms, but decent sized bedrooms over tiny bedrooms + an en-suite.
