Agreed that checking for incompatibilities is valuable, but IMO there’s value even without that – one separate advantage to something like poetry add is minimizing context switches (I don’t have to open a separate editor buffer, jump to the right TOML table, etc.).
IIRC that’s what cargo add is limited to: pretty much all it does is modify the TOML, and subsequent commands are expected to actually perform the resolution (and fail, if appropriate.)