I don't know many, if any, guitarist is not looking for something extra, something new. Maybe some are not happy to gig a 'core' model because of the price or fear of damaging them but they may still want a core (or better) for home/studio use or just having their 'ultimate' guitar. There is always a reason to buy a 'new' guitar and each can be justified by a musician some way or another. Its not always about having something 'better' either but maybe something different - something that may inspire them or just something they 'want'.
The 594 is, in my opinion, something 'different' to anything PRS makes. I know they may look similar to other guitars - the standard 22 fret, twin humbucker format - albeit with a shorter scale length, thicker body and classic Les Paul style arrangement which isn't 'common' - certainly not in their current core line-up but its much more 'vintage' inspired than 'modern'. If you want a 58 Les Paul, you could by a re-issue or buy a 594 if you prefer PRS guitars. It may be similar to what you already have but it may also be different enough that you can justify it. I think the neck carve is the best that PRS offer as standard and really fits the 'vintage' feel they are going for.
I do think that the SE's are great - probably the 'best' in their price bracket, good enough for professional gigging musicians which is high praise and shows how great these are but the 'core' models offer something 'extra'. Maybe its only an extra 5% which may not be a 'lot' extra for the big jump up in price point - some have more for example like the Custom 24 has 66% more options having a 5-way as opposed to the SE's CU24 3 way - but whether that's 'enough' for some is down to individual situations and preference. Whilst a lot of 'signature' guitarists may tour with their SE's, they do have Cores, Private Stocks etc at home too.
If you want something you are happy to gig with but worried about gigging such an expensive guitar, stick with SE's but if you want something special then go for the Cores (or better). Its like owning a Ford Fiesta for running around town, a Ford Pick-up/Van for work/gigs and having a Ford GT in the garage for those special occasions when you want something 'extra'. It maybe that you would rather have 2 or 3 extra guitars for gigging and can't justify having that 'Ford GT' because you never get the opportunity to use it because you are always working/gigging. Only you know if its the 'best' use of your money. I do love my 594 though and wouldn't sell/swap it for anything....