Why would we want Tresco to keep? As a reference take Alec Stewart. Averaged over 40 with the bat as an opener, averaged around 35 by the end of his career once he took over the gloves.
Tresco is one of our most consistent performers with the bat, and as Sureshot said the best slip fielder we have. To have him trying to balance both keeping and opening the batting could be highly detrimental to his game, and considering his importance as opener over the last 4 years, that would be a massive gamble to take.
Not to mention that as a keeper, he is almost certainly not as good as Geraint Jones. If he was, that would reflect extremely poorly on Jones considering Tresco is a part-time keeper.
For me Geraint needs to produce consistently with the gloves for the rest of this series v Sri Lanka, and since he will play against Pakistan barring injury, for that tour as well. He has a tendency to have a good game, and then follow it up with a poor game. If he can average over 30 with the bat again that would be nice too, but at the least he has to start being consistent with the gloves, at the most refind some form with the bat.