Plugins and hardware units are not the same (even if they have the same name.) It's mainly a matter of personal preference and what works best for you.
I'm a little skeptical of a $300 program that claims to perform as well as a $1500 box but maybe that is just me. There are definately great plugins out there and like all things you get what you pay for.
It would seem that most mastering engineers prefer digital limiting and analog compression and EQ. But the bottom line is always if it sounds good it is good.
I would also agree that an external compressor is one of those things that is near a must have for a DAW. It's not that you won't use compression plug in's for some stuff but I swear my RNC gives me a sweet compression on vocals that I can't get out of a plug-in.
I know people who would argue the same about a nice reverb as well.
In my opinion, the effects in the UAD-1 are pretty damn good!
But I`m still hooking up my RNC to compress lead vocals......
And Iwould take a real 1176 or LA2A any day over the UAD-1...
I have experience with the originals and they are just fantastic!
I like DAW all fine and great, but theres something to be said for the feel of "live" effects as you play. For me I like hearing the delay or volume sweeps that help me with alot of my strums or string chokes.
Personally, I like using plugins. I have a UAD-1 in my Nuendo rig and I use it on all my projects. The LA2A compressor if just fantastic for vocals and output bus limiting.