Aerocat has it in one. If the other guy is obviously on it, no need for a call at all. On the other hand, if the speed is trending the wrong way and the PF is doing nothing about it, the 'speed' call is warranted. Ditto any other divergences - it's the timely corrective action you should be looking for.
Once while doing a base check in really gusty conditions I had the anal checkie next to me yelling 'speed' about every 10 seconds as the ASI was bouncing around between Vref and about Vref+20 - never mind that the average of all this was a nice safe Vref+10. Rather than blow the check by telling him to shut his cakehole, I said to him: 'your control' then about 10 seconds later at the first 10 knot divergence yelled 'speed'.
I only had to do it twice more for him to hand it back to me and complete the rest of the detail in total silence.
Gratuitous calls are NOT good CRM; in fact the very reverse.