Having the recovery seed is better than having private keys.
You CANNOT import private keys into a HD wallet, it's just not possible for it to be useable or storable and would require another wallet that is non deterministic.
When importing a private key, it's a good idea to sweep the key so you don't have to keep that key stored somewhere for you to get it after. Sweeping does cost you in fees though.
Could you please elaborate on why seed is better than a private key in case of a single currency wallet? I don't really see any difference.
