i think it's a cunning game played by Zardari, they already showed how easy was it for them to win senate election and wining this would not be a rocket science for them. If they had won IK would have resigned and moved to opposition benches and in next election he would have used that "i never completed my term" card like other and all bad governance, decisions and what not would have white washed behind that lame excuse, now that exit strategy is thrown out.
lets think out of the box and consider possibilities and don't take this cunning opposition for granted
A problem with your comment: Even if PDM showed up and voted, PTI would still win the no confidence vote, because PDM doesn't have the required numbers.
This boycott was A) a face saving measure, because PPP has said they want to push for a no confidence vote a few weeks ago, but PTI beat them to it. B) it was a method to make sure to avoid embarrassing themselves in case some of their own dissenting lawmakers voted in favor of PTI. C) It was a way to try and make the process look anti-democratic, because the opposition wasn't participating at all.
Also, PTI won the biggest share of seats in the senate elections, so it was certainly not easy for PPP, even when PPP were very blatantly rigging the voting process.