Originally posted by BrianR You don't have to conform to any sort of organization with Lightroom, however it does need to maintain a catalogue of all the photos you want available in it.
+1
There is quite a bit of misinformation in this thread but BrianR's post is spot on.
There is no reason why you cannot keep exactly the folder structure you have and you don't even have to use LR's import dialogue because you could just add images manually to your existing structure and then "synchronise" LR's catalogue to reflect the changes (and thus achieve an "import" of any new images).
Having said that, LR always uses a catalogue with abysmal inefficiency -- the file will grow to insane sizes because of the atrocious redundancy recorded and the inefficient format used. Compressing the file would be easy but until LR 5 I know Adobe did not do it. They may still not compress the catalogue file, I simply cannot comment as I stopped following the LR development as soon as I realised that Adobe cares more about product integration (tablet, cloud, etc) and dumbed down, non-optional "auto"-functionality than about giving one a serious RAW converter.
Unless you maintain several catalogues, you will be putting all your eggs in one basket. Catalogues can develop problems and if you have only one, "fun" with this is almost guaranteed because even when you backup regularly (say "good bye" to a lot of hard drive storage unless you compress manually, e.g. with WinZip) then you are not guaranteed to find out about a problem straight away and would have to unroll quite a bit to find an uncorrupted backup.
I've had catalogues with problems that slowed LR down. It would have been a nightmare, if that had affected all my photos (I maintain several catalogues).
Also, if you change the file structure outside LR (and that could just be a change in drive letter for an external disk), you'll have to tell LR to where things went, otherwise it will not be in sync with your structure anymore.
Perhaps try Capture One 9 as an alternative organiser with a really nice RAW converter that may save you many trips to PSE. CO9 can work using catalogues, but it also supports a catalogue-free approach it refers to as "session-based". It pretty much works like Adobe's Bridge with this approach.
You can call external editors like PSE from CO9 and while I haven't fully evaluated CO9 myself yet, I like a lot what I'm seeing.