I saw a Gearslutz forum discussing this very subject recently...
What I took away from that one was this: Logic's plugs are great & you can totally get the job done with them but what other 3rd party plugs offer is either something specialized or convenient in the form of design.
A good example of this is what I found with my new UBK-1 from Kush Audio...
This Compressor/Limiter/Density-Enhancer is pretty amazing right outta the proverbial (and virtual) box.
I don't know how many of Logic's Comps, Limiters & such you'd have to route, bus and stack just to get what one UBK-1 plug does in just one instance... Not that it couldn't be done, but I'd rather just dive in and quickly make some music without getting too weighed down in plug-in manipulation, (at least early in the creative process...)

ANother one I just got is the klanghelm DC8C which is a Master Bus Compressor (or anywhere it works,

) which has incredible input range with very little harmonic distortion (and it's super cheap... Like $30 kinda cheap.)
Nomad has some really good analog modeled Comps, EQs, vintage warmers and delays as well... I'm a fan of their Blue-Tube compressors for tracking anything.
So these are great for my needs and immediate because they are designed with a certain set of styles in mind, right off the bat. That convenience is handy when inspiration strikes.
After that there are the specialized plugs that I love like the Soundtoys suite (which I use on everything) and some of PSP's delay/mod effects which are great as well... iZotope's Stutter-Edit is amazing even if it's a bit counter-intuitive to make work in Logic. But it sounds amazing for what it does.