The argument seems to revolve about prioritising and protecting the live event weighed against the outreach and access afforded by the cinema experience.
In respect of the aesthetic, my own experience has been largely positive. When I saw the Royal Opera Macbeth last year, I appreciated the opportunity to see the production close-up and to have an experience (relayed live, with an audience in a darkened auditorium) akin to that of being in the opera theatre.
Naturally, I missed the physical connection with the performers (although that's not a privation in the other direction, i.e. the performers do have a live audience to whom they are performing) and one is also aware of having one's attention drawn in the direction of the relay director, rather than that of the stage show director.
As for the access and outreach issue, I regularly attend opera productions in and around London, so this event as an advertisement for the artform is not relevant to me (preaching to the converted, if you like). I was attending largely to assess the experience of opera in this relay form.
It so happened that on this occasion I spotted someone in the cinema audience related to one of the performers who lives quite close to me and the cinema we both found ourselves. This seems to me to suggest that a cinema relay provides convenience for those who are interested in seeing the production for whatever reason but are prevented by distance or timing.
At £25 for a ticket, the cost issue is a complicated issue, if not an outright red herring. You know you will get a good view in the cinema for this premium and this 'premium' is also the bottom end of what one would pay for the live experience. It's still not the live experience though. Central to the functioning aesthetic of opera is the physicality of live singing and no mediated access will succeed in replicating that.
In short, the cinema relay provides yet another facet to the way in which one can discuss opera circumstantially, yet it still remains outside the experience and so a tool for experiencing it rather than a representative substitute.