[darklajid]

Needs a G+ login, so it's inaccessible (and as others wrote: Probably wouldn't work on a rooted phone anyway).

[thescrewdriver]

Nothing worse than systems which force you to use a Google account to log in.

[RoryH]

Lack of support for rooted devices probably rules out a whole lot of potential first-adoptors

[kiiski]

I'm not very well versed with Android related things, but isn't a Google account required for Android anyway? If so, using it to sign in to an Android app seems like a reasonable choice.

[ZoFreX]

> isn't a Google account required for Android anyway?

Not strictly speaking, no. You do need to sign in to a Google account to access the Play Store, which is synonymous with Android for most people, but it doesn't need to be a Google+ account.

Even given that though, the vast majority of Android apps don't require you to sign in with a Google account (or a Google+ account) and thus don't have access to the data associated with it. For a lock screen app to even require signing in is pretty strange.