Why? Bag and baggage, the private keys only matter. Is there any essential difference in private keys generated from BIP39 SEED and native Electrum SEED? The procedure to create the legacy wallet via Electrum GUI is much easier than via its console and eliminates human errors.
Honestly, if you need a legacy wallet, there is simply no reason to choose BIP39 over Electrum seeds. Electrum seeds have obvious advantages over the former and generating it from some other sources raises some security concerns as well.
The story changes with nested Segwit wallet (which I think OP should use over legacy). There's no nested Segwit for Electrum seeds and for which BIP39 would be your only option.