There is a lot of movement between march and may, after that it begins to slow down. That being said, because of their hold list size, unless you make the top half or even quarter you're not getting accepted this round. It seems to me like a "we're interested in you for the future" deal if you make the hold list but aren't at the top of it.
edit: and yeah IIRC its around march 15th that schools see people holding multiple acceptances, HOWEVER, I don't think schools get to ever see where all you have applied and are ONLY privy to your acceptance info if they themselves have accepted you
So, I think it is more for recruiting scholarships. If you have one acceptance, hold it. Because after May 1, people will have to start deciding where they are going and the picture is a lot more clear for schools accepting ppl off the waitlist late.
Apologies if this was confusing, hope it was some help.