I know they're 9 down now, with only 10 and 11 at the crease, and declaring over lunch would avoid the loss of two overs being deduced, but I think Somerset would be mad to declare now.
They've got us totally on the floor. An injured bowler, miles behind on runs, leaking runs all over the place. Every extra boundary they hit, every extra over they force us to bowl, is just another kick at us. Sap our confidence even further, going into our second innings (an innings which has now moved from setting Somerset an interesting last day run chase, to just trying to occupy the crease for long enough that they won't be time for a last day run chase - because right now the match will be over before tea tomorrow, with either an innings defeat for us, or an exceptionally straight forward run chase for Somerset).
OK, they may lose their 10th wicket with the very next ball bowled. But at least try and smash us in even more.