How can we calculate the benefit gained from taping glider wing roots?
The two questions actually concern very different issues.

The second question is easier to answer. Let's take your thought on:


  I've always wondered though how critical this practice is.


It is pretty crucial! If you did not tape the gap between the wing and the aileron, you increase chances of a stall. This is especially true at low speed and large angles of attack, i.e. while thermaling or landing. Then an airflow from the lower side of the wing to the upper side might occur, through the gap, destroying the upper side's airflow. This very certainly leads to a performance penalty but also favors stalls. There have been some prototypes experiencing exactly this loss in performance (I could provide more detailed references if desired). I haven't heard of a stall related to this issue, and it actually might be unlikely to happen because under the described circumstances the transition point from laminar to turbulent flow is rather close to the wing's leading edge anyway. Nonetheless, the possibility of increased stall probability should be considered and, imho, is reason enough to never ever fly without aileron tapes.

The first question can only be answered qualitatively, at least with today's methods. It is effective: the airflow, although transitioned to turbulence by the tape, is definitely less turbulent than without the tape. Any kind of turbulence model will consider the "amount" of perturbation, i.e. larger discontinuities of the surface lead to stronger turbulence, thus more drag. You could fly with only one wing taped and see if you can notice a difference. If you notice rolling or yawing, then that's in favor of the corresponding wing. Honestly speaking, I doubt anything can be noticed if the wing consists of one part only, because the asymmetric drag is so close to the center of gravity (only at wing root). But if the wing is made of multiple parts, that might actually lead to some practical result. I am thinking of, e.g., Schleicher's ASW 28 or ASG 29 that have inner and outer wing parts + winglet, meaning 3 tapes distributed over the half wingspan. However, please check the manual of the glider first, if flying without tapes is permitted by the manufacturer.

And then there is also the noise issue.

Given all this information on the one hand and comparing it to the cost of tape and the time it takes to apply it, there is no reason not to use sealing. (Except, of course, around the canopy. You wouldn't expect what kind of ideas people can come up with)