Any particular reason?

Not trying to be argumentative, but what are you trying to accomplish by pressing the record button, but not sending/saving the audio anywhere?  Wouldn't play work for this?

I would guess that if you've got a number of hidden or off-screen tracks armed and you start accidentally recording layers and layers of audio over them then you'll be filling your hard drive and possibly pushing your processor over the edge.
MIDI data is pretty simple stuff that uses kilobytes rather than megabytes.
 
Just my 2 cents...

dwardzala
Not trying to be argumentative, but what are you trying to accomplish by pressing the record button, but not sending/saving the audio anywhere?  Wouldn't play work for this?

I believe the OP is trying to accomplish simply a spontaneous recording. The question does not include any speculations about what is routed/recorded where, but simply the idea of being able to hit "R" on the fly and record on the track in focus.
 
I'd consider it a somewhat risky feature. I believe it's good we don't have that possibility. There's so many kind of damage I might do with it ( but I'm a pessimist  :o)

Data size is part of it. MIDI is easy because the recorded data can go right into RAM with no preparation of a physical storage medium. Audio has to have a file opened for writing to disk. Also, I suspect that the handshake between the DAW and the audio interface driver to open the port for recording (especially when input echo isn't enabled) is more complex (and slower) than for MIDI; this would slow down moving focus from one audio track to another and could lead to issues with manual punch in/out, changing track focus when the transport is running, etc. Throw in the variability of audio interface driver/firmware/hardware behavior and performance, and you could have a real can o' worms on your hands.

I think Brundlefly pretty much nailed it. One of the very few things I miss about tape is the ability to hit the "record" button at any time.

Well if you right click the record button and select allow arm changes during playback/record, you almost have this. However SONAR doesn't let you start recording unless you have at least one track armed. You can however disarm that track after recording has started - so in theory we could allow the transport to roll in record mode but only start recording when something is armed (as we do already in dynamic arm mode).
 
Starting recording with nothing armed could be a bit confusing though since you could go through a whole pass thinking something was being recorded and find out you got nothing :)

Thanks for the replies.

If you mostly do audio recording and don't record MIDI very often (and if you have the "Allow MIDI recording without an armed track" option disabled), I guess you almost always arm the track first before hitting R to start recording, so the steps to record are always the same between audio and MIDI. But for someone like me who mostly record MIDI (haven't done any serious audio recording in Sonar for some years now) and is accustomed to the convenient "record without an armed track" behavior, the steps to record in audio tracks one by one (i.e. having to manually enable and disable arming each time) seem somewhat a hassle.

I think it's best to drop the "record without an armed track" feature and then add an auto arming feature (as an option) to both audio and MIDI tracks. And, of course, the previous track that was auto-armed should be automatically disarmed. The reference guide says Sonar doesn't allow auto arming because you could unintentionally overwrite (delete) an existing audio clip by mistake. But I'm not sure. When you record into a track, the first thing you do is click the track to make it a current track, isn't it? Also, we have Undo, don't we?

And, ironically, the current implementation made me unintentionally overwrite a clip in the previous track because I FORGOT to disarm the track (used Undo afterwards, of course ). Another daw I use has auto arming for both audio and MIDI and it works beautifully, so I believe it's technically possible. Maybe I'll submit a feature request, if not already done by someone else...