Here's something: There have been found a Triceratops fossil with skin impressions. This Triceratops sp. "Lane" was found at the Zerbst ranch in Wyoming. Among this impressions are structures which may have been quills or protofeathers (yes, no prove but a reference point). Alltough Nima's reconstruction is very speculative, I agree with you about the limited size of such attachments.
If it had them, they surely hadn't an real protective effect I think. Thermoregulation would be nonsense too on this big animal. But I think it could be used for attraction and/or deterrent because the hindquarters seem to be the less protected part of its body. The quills I depicted are arround 150cm, rather shorter, and fulfill the such an effect. But of course this is purely speculative since we have no proof for or against the "quills".
You know, I'm a friend of innovative ideas in Palaeontology if they are plausible.
I think the "problem" is not the lenght of these quills, but their diameter. On a 9 tonnes full grown Triceratops, their diameter should be too thick, not flexible, and then a fragile structure. I don't think Triceratops – or any other big Ceratopsid – had these quills, but I respect your own view.