You're right to say that the complementarity question is a difficult one - but it's also an important one, and I think it should be retained. LIFE+ is a relatively small instrument and needs to be focused on problems that can't be addressed through other, much bigger instruments.
A crucial point is that the people who evaluate LIFE+ proposals seem to take a very sensible and pragmatic approach to this. What they look for is evidence that actions cannot be funded through other instruments IN PRACTICE (rather than in theory).
For example - in the UK, a huge range of work can be funded through agri-environment schemes in theory - but only a small proportion of this work is funded in practice, because of the way in which the schemes are implemented "on the ground". The evaluators quite rightly want to know what money is actually available under the schemes, rather than what might be available if the schemes were infinitely large. They understand that, if you need spend £1 million restoring active blanket bog in an SAC in north Wales, the fact that you might be able to get £20,000 from the Welsh agri-env scheme isn't terribly helpful!
In response to the bit of the question that asks whether there are any cases in which LIFE+ is the only option - the answer is yes, lots! In fact, almost by definition, everything that's been funded under LIFE-Nature and LIFE+ Nature to date falls into this category. More specifically, there are lots of habitats and species that aren't well covered by agricultural or fisheries funds, and lots of areas that aren't well covered by Structural or Cohesion Funds. (And in any case, most other instruments require that projects have clear social and/or economic benefits - LIFE+ is just about the only one that will fund "pure" biodiversity conservation , for its own sake. This is hugely important!)
Examples of LIFE+ Nature projects that I know about and that couldn't have been funded through any other instruments include the blanket bog project mentioned above, a project to conserve machair in western Scotland (which, interestingly, aims to test and develop NEW agri-env options so that its impact can be sustained and built upon in the long term), and a project to carry out major civil engineering works to protect important freshwater habitats in an SPA threatened by coastal erosion and sea level rise.