I remember experimenting with the RTA function when we first got the aircraft and found the speed went up and down a fair amount as small changes in wind, temperature, etc. fed their way back into the sums. OK if there is no speed control but if ATC are trying to maintain separation it gets them on your case fairly quickly as Mode-S tells all...
Overall, it seemed easier just to pick a speed and revise it occasionally, rather than having it hunt all over the place.