As always: it depends :)
Because so much of the internet is still IPv4-only there is a kind of implied responsibility to have IPv4 connectivity yourself. There are however advantages to having both.
The world has a huge shortage of IPv4 addresses, which means that there are a lot of hacks like carrier grade NAT, NAT64 and DS-Lite going on to keep users connected to the IPv4 internet. That means that when your service is only available over IPv4 then your users might be forced to use those hacks. When the service is available over both IPv4 and IPv6 the user can just use IPv6 and get better connectivity to your service.
This is probably also the reason that big services like Facebook and LinkedIn see 15% to 25% better performance over IPv6: those users don't need to go through a central NAT box at the ISP and don't lose performance there.
And if those central NAT boxes get overloaded (they shouldn't, if the ISP invested in enough capacity, but...) then IPv6 users won't even notice.
So I'd argue that there is already a certain availability advantage to deploying IPv6, and it will become a bigger advantage every month. In countries like Belgium almost 60% of the home users already has Ipv6 access.