It depends on where the person is that you're onboarding. If they're absolutely new then going into seed backup + 3,000 sats fees per few receives can very easily get in the way.
The point of onboarding is just getting them into the ecosystem, understanding this new unit of account and new paradigm of sending/receiving via lightning/onchain.
Next steps can then be self-custody (by sternly reinforcing "not your keys, not your coins"), but the familiarity of being able to access funds via email with Wallet of Satoshi is a stickier first step from my experience.
It's something I had to consider for example, for onboarding events like these:
I guess that with wallets and with a small amount of money it's not the same, but we also said to the people "start with FTX, then you'll learn how to use your own node" :)
But yes, I 100% understand what you mean.
Yeah, you also get inbound liquidity in the same amount. I'm not sure about other wallets (that actually open a channel), but inbound liquidity is one of the most important aspects of using LN. In general, you pay more (than Phoenix fee) for services that provide inbound liquidity, so that should be considered as a feature, not an issue.