Until Google implements the behavior and applies it to their sites. Then Google services stop working smoothly on Firefox and Safari because the shared session on those sites is only functional on Chrome. Imagine having to log into gmail, calendar, drive, etc. individually each time. In theory it's not that big of a deal, but in practice could end up driving the casual users to Chrome since the tools they use work better in that browser, and ultimately they don't care which icon they click to check their email.

And it'll be worse if other services decide to apply the new cookie strategy to their services, though they are definitely less inclined to do so than Google would be.