By themselves, the phrases "except for" and "other than" have similar
meanings. In sentences, both of them make a noun phrase, where "___
aesthetics" modifies "reasons":
(b) they actually were designed for reasons except for aesthetics.
(d) they actually were designed for reasons other than aesthetics.
The noun phrase in (d) is okay, but the noun phrase in (b) is not okay.
Note that (d) is similar to:
The round windows are indeed pleasant to the eye, but they actually were
designed for other reasons.
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