To be or not to be

An interesting observation came up in a greek reading group yesterday. We were reading in the sermon on the mount at Matthew 5:48. Here is the Greek text:

ἔσεσθε οὖν ὑμεῖς τέλειοι ὡς ὁ πατὴρ ὑμῶν ὁ οὐράνιος τέλειός ἐστιν.

The question is concerning ἔσεσθε which is a future middle indicative 2nd plural from εἰμί.
Many translations treat this as a command or imperative with, "Be ye, therefore, perfect...,"
but should it rather be treated more as a result? "Ye, therefore, will be perfect..." or even "You, then, will be perfect..."

Are there any syntactical or grammatical reasons for treating this future middle as an imperative?
Commentaries I've looked at so far seem to assume one treatment or the other without dealing with the grammatical question.