"Glassing" is a really serious offence and there have been guidelines specifically relating to it.
The original suggestion was that a deliberate glassing should attract a tarrif (time actually served) of 3 years and I think that was upped to 5.
So it's pretty good that they accept that it was accidental and he was doubtlessly charged with ABH rather than GBH because the injury was quite slight.
However the fact that it's being heard in crown court may indicate that the magistrates do not feel their powers of 6 months imprisonment are sufficient and in the sentencing guidelines consultation document use of a weapon in ABH has a *starting point* of 12 months (see below)
http://www.sentencing-guidelines.gov.uk/docs/A SSAULTS-Consultation-paper.pdf
An unplanned offence with minor injuries however attracts only a community order
I suspect it will come down to whether they truely believe the weapon was an accident - sounds like the sort of situation where a good lawyer could make a big difference