If they are truly only in the aerobars when they are on the front (your words) than it seems pointless to be on a TT bike because the rest of time they have to be on the horns which doesn't make alot of sense due to the fact that the overwhelming majority of time will in fact be pack riding. My guess is that they are in aero more than just on the front. People don't ride a tt bike to simply sit on the horns for a over whelmingly majority of a ride. That basically compromises the advantage a TT bike gives a rider. I've seen too many rides to know that people are in the aerobars for more than just the front pacing.... Maybe it is different for the pro's when riding in a public group ride (you already said they only ride in aero on the front). So my point is, it doesnt seem to make alot of sense to ride a TT bike in a public group ride if your only going to use the aerobars when on the front. Lets say there is 15 people in a group ride, by what you say, each rider will only be riding in the aerobars 1/15th of the time (basically the pull that they are doing while in the paceline rotation, if we want to "average" it will probaly be less than 12-15% of the total ride where you state they are in the aero=being at the front). Which to my point, my guess is that people do in fact ride in the aerobars for more than just the front (and not counting when they get chewed off the back).

So my question is, why even ride a TT bike when your only going to be able to utilize the aerobars for a small portion of the group ride?