DuckJordan
Touring IATSE Member
A Google image search will return all manner of pictures and articles that explain SSR/SCR/Triac behaviour and what happens around the zero crossing. This article is pretty good. The section titled Solid State Relay Output Waveform covers some of the issues. It doesn't cover the impacts of an inductor in the circuit.
Frankly, your bidding committee likely isn't qualified to make a decision about whether to use a relay or a dimmer. The simple answer is "because the manufacturer requires it". The committee could hire a professional engineer to make such a determination but that would cost more than the relays and give the fixture manufacturers a good reason to void the warranty.
As much as I'd love to say that the "committee (one gentlemen in charge off city assets)" would hire a professional engineer to explain a better solution. Its doubtful we will be able to convince him that adding on a $3-$4k consultant fee is going to fly over well. I'll read over that article and see if I can't get a good solid basis as to why using our D20 modules @F is not a good idea for either our vipers or the planned LED upgrade for our halls.