You should always, always control your secret key. A seed is only meant to recover your Bitcoin in the case you forget your wallet password. If you have your secret key you can access your Bitcoin anytime, anywhere whereas the seed only works for the particular wallet client you are using. Makes sense?
That isn't necessarily true... there is some commonality between wallets... like Breadwallet, Simple Bitcoin Wallet and MultiBitHD all used the same Derivation Path (m/0')... Most hardware wallets are all standardised on m/44'/0'/0'... and even some software wallets like Mycelium use it as well... and now that Electrum supports custom Derivation Paths when importing BIP39 seed words, it can import from pretty much any wallet. Also, as the method for converting Seed words into private keys is public information, there are plenty of tools (like: https://iancoleman.github.io/bip39/) that can convert your seed to private keys using any derivation path... So you do have options.
Private Keys are derived from the seed... so if you have the seed, you can always get your private keys... it might just take a little work
