The only rationale is that it is pretty easy to build a cw rig.
It is only slightly harder to modify that rig to do DSB.
It is very hard to build an SSB rig.  

Homebrewing an SSB rig is beyond many QRP experimenter's ability 
so you either have to resort to a commercial kit or a commercial rig.

I imagine as time goes on, more and more hams will be able to 
do digital modes and less and less hams will be able to do CW
(unless they treat CW as a digital mode).  



I have modified my Heathkit HW8 to do DSB.
I would like to try to make a BPSK contact with it.

My thoughts reguarding QRM are these:

1) Since the HW8 is a direct conversion receiver, I would hear 
   any stations on the wrong sideband and so avoid QRM to them.

2) If someone came back to me on either sideband I would be able
   to answer them if I was using BPSK which doesn't care about
   polarity.

3) If I use BPSK, my total bandwith consumed would be 2x31HZ = 62Hz
   (if properly adjusted) which is not as wide as many other 
    digital modes.   I think I would avoid the wider bandwith modes.

4) If I kept my audio frequency low (say at or below 1khz) then
   the primary sideband would be at 14071 other sideband would be 
   at 14073 which is still in the "psk subband".  If I tuned my vfo 
   to 14069.5 and kept the 1khz audio freq then the primary sideband
   would be at 14070.5 and the other sideband would be at 14072.5
   which is well within the psk subband.

I realize the potential for QRM.
At QRP levels and being able to hear any qrm that might be caused
seems to me to mitigate the problem.