You can access our services only through a Google Account. Why? Because virtually everyone has a Google account and this way you don't have to remember yet another password.
I see the logic in using OpenID for signin. But only google?
Defi9nitely not everyone has a google account (though perhaps your target audience will have). Also what about the people that do not want to use Google's services (there are plenty). And if you dont have a Google account it's a pain to go elsewhere - sign up - then come back.
However.... www.clickpass.com will do exactly the same thing and offers a much wider range of logins :) (HN here uses it). YOu might want to look into that - I doubt it will take much to switch to using Clickpass (as the openid sequence for them is similar to Google I believe)