B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 150; Flight Crew Type 13000
ATC Issue All Types
Air Traffic Control Issued New Clearance
Procedure
On the ground; we were cleared to climb via the SNSHN 1 RNAV Departure by Tower except climb and maintain 14;000 FT.  On departure shortly after the POM hold down fix of 8;000 FT; SoCal directed us to change the transition from the filed and cleared LAS transition to the BEALE transition; and after LIVVI fix; cleared direct YELAH then climb and maintain 14;000 FT.  We were at a low altitude; I've never heard of changing transitions on the departure; we don't have time to load; execute; brief; and verify the new clearance prior to the branch point only seven miles away; so I said; 'Unable.'  ATC then step-climbed us to a higher altitude.  ATC later said another flight was able to do the change; and I said we didn't have time.  I say good for them.The next Controller did the same thing; directed us to another fix on the departure; but since we now had time and were above 15;000 FT we complied.  I asked that Controller why the transition changed.  They said they were using them to deconflict traffic.  I asked; 'Why didn't we receive the other transition on the ground?'  He said they didn't know about the traffic then and that they were using transitions to deconflict on the fly.   I asked; 'If I took the other transition; how high would we have been cleared?'  He said 14;000 FT.This needs to stop ASAP; no pun intended.  We go to great lengths to file; load; verify; brief; and execute the clearance. We take many minutes to setup on the ground in a stationary environment and cannot expect to; in the heat of the moment with two minutes to go to a branch point; change transition just to make it easier for ATC.  This is a downright dangerous action by ATC.
B737-700 Captain reported he was uncomfortable with an ATC request to modify an ONT departure procedure during the climb; stating the increased workload at that time would be a safety issue.