Merging data segments produces smaller code sizes because each segment has some
boilerplate. Therefore, merging data segments is generally the right approach,
especially with wasm where binaries are typically delivered over the network.
However, when analyzing wasm binaries, it can be helpful to get a conservative
picture of which functions are using which data segments[0]. Perhaps there is a
large data segment that you didn't expect to be included in the wasm, introduced
by some library you're using, and you'd like to know which library it was. In
this scenario, merging data segments only makes the analysis worse.
Alternatively, perhaps you will remove some dead functions by-hand[1] that can't
be statically proven dead by the compiler or lld, and removing these functions
might make some data garbage collect-able, and you'd like to run --gc-sections
again so that this now-unused data can be collected. If the segments were
originally merged, then a single use of the merged data segment will entrench
all of the data.
[0] https://github.com/rustwasm/twiggy
[1] https://github.com/fitzgen/wasm-snip