On the 737-3/4/5, if the FD is on with no lateral or pitch modes pre-slected, and you engage either autopilot, it will default to MCP SPD and HDG SEL.
As part of Special procedures which vary from our outfit's normal SOPs - departing certain CAT C airfields such as CBY and INN, in order to afford increased situational awareness, Autopilot use is maximised. We can therefore select an AP at 500'R after take-off, and all you have to do is select the autopilot and adjust the speed in the MCP Speed window to Top-bug - And there you are......barrelling on up initially at up to 6000fpm!
So...just to recap, in the above situation you get, after AP selection, FMA annunciations of TO/GA, MCP SPD, HDG SEL & CMD.
If No FD selected on then as others have mentioned you end up with CWS in both pitch and roll.
The only downside (on the question of Stability) is IMHO the 737-3/4/5 autopilot is quite frankly crap in pitch, especially in FMC/MCP SPD, and lags quite badly in correcting to the commanded speed - (it seems worse on the 300 and 500 than the 400). In the aforementioned situation, I've found it necessary to adjust the speed in the MCP window a good few kts above or below what i really want in order to coax the autopilot into adjusting pitch in an, *ahem* ...more timely manner than it would like to.