Then why aren't there more classic homes around? The majority of homes by far are custom, and even if they are three story boxes or even bare plots people seem to prefer them in general. Definitely tho a nicely designed home will sell for more, but selling houses I think is more about knowing your market.
Location and size are more important to people than the actual house nowadays. If they care about how a house looks they can tear it down and throw up something else or recustomize to their own satisfaction. If they don't care then they can live in a dirt plot or a tower and be just as happy. Rare is the person who cares about what their house looks like but has no inclination to do anything about it, and even then those people usually consult with others or copy another design to get what they like.
Don't get me wrong, I think large towers are great. They aren't space hogs with poorly designed areas like keeps or castles. But they no longer are the status symbols they once were, now that anyone and their cousin can get a customizable house that is larger in size. Only keeps and castles trump full size custom houses nowadays, and both keeps and castles suffer from poor design, both in wasting space and being difficult to get around in. The flexibility of custom houses is immense.
Having said all that, of course the best option in my mind for a busier shard is to have a max custom plot with a lovely design, and maybe a lil deco. It's not too much more depending on what you expect to make from the plot, and you can always snag that one person who can't design for **** and thinks that the house is too awesome to not have. On a less busy shard it may not be worth the gold to put into customizing, as you *might* be able to sell for 500k after combing the whole shard for a buyer and the price really won't budge from there, customized or no.