A mixer is useful on the front end, getting sound into SONAR. In that case, you use the analog mixer's controls for gain, headphone mixes, etc. Either for a multi channel mixer or analog mixer going straight into your interface .
What Cactus is referring to is about mixing in-the-box. Cakewalk does excellent clean mixing, along with full racks of digital effects to put on those virtual tracks. And automation. Good automation for analog mixers is a hardware retrofit at $5000 per 8 channels. In SONAR, you can mix 80 channels with automation on each vol, plus pan, plus whatever else you want. That is a strength of any DAW. Of course, if you want to mix through the analog mixer you can do that, too, with as many channels as you have digital to analog converters.
The sweet spot for many hardware people is a summing mixer, which is basically a line mixer. You shunt out 8-24 channels (however many D to A converters you have) into a simple mixer, sending tracks to your analog hardware units. Such as your DAW bass out DA 1 into channel one of your mixer. That signal then shunts off to your 1176 Limiting Amp and back into the mixer (either through a line insert or just another unused input). You can either record the return signal in your DAW and move on to the next instrument, or mix live with it still attached to your DAW. In that case you find the next signal, say a vocal, and use an LA2a on. Your DAW still supplies automation to the tracks (as well as any digital effects including comps and limiters) while your lead tracks get the hardware polish.
It is all a balancing act, tho. A $200 mixer that you use for headphone outs during tracking isn't going to add anything special to your sound - in fact, it is as likely to muck up things if your stereo interface costs as much. Mixing back through it for eq etc. is a waste - your digital EQ is much better (esp. if you are using SONAR with the ProChannel). A quality transformer-based preamp, or comp for that matter, is going to cost $400 a channel to start out at and only go up. It gets expensive quickly. Your best bet is to build up your hardware - a nice stereo comp is a great buy, since you can run a signal in through it to record, then run your master out through it when mixing. A two for one deal. Same for EQs. And a high quality pre can really help with the definition of various instruments in a mix. The best thing that ever happened to me for acoustic recording is fall into a deal on a RND Portico II channel strip. At that level, if your signal sucks, you know it ain't Rupert Neve and must be operator error.