I think the rebuilding job over summer, if we got promoted, would be so big that any players we can tie down on the cheap will free up money for other areas of the squad. If we can get him cheap, or even free, it's one less problem. Not just from the financial point of view, but he's settled at the club and if we buy someone else they're a gamble, need time to settle in etc etc.
If we went up we would simply have to invest in a striker, a creative midfielder, a winger, we basically need a bigger goal threat and those players don't come cheap. Adding a right back to that list is an unwelcome headache. Simpson is pretty limited and the Premier League would be a big step up, but I think it's a case of beggars can't be choosers.