How necessary is SPI in reality, for a home router?

I've managed using a router without it until now - is it just a way of
selling me something that I don't need?

Hence what's the threat/risk, if any, to:

- my router h/w?
- my BB service

I'm a VoIP user and there's a trade-off in choice of ATA/router if SPI
is something I should consider.

TIA