If one (or several) elected MPs fail to deliver promises, it doesn't follow that all 650+ need to be re-elected or removed. Fixed-term parliaments prevent disruptive and costly extra elections at a time when economic recovery and stability are in the interests of both Scotland and the UK as a whole.
The 55% threshold for dissolving Parliament is actually quite low. Scotland's threshold is two-thirds. Other countries have similar barriers to hasty and petulant dissolution.
Electorally, the five-year span is more surprising to me. Scotland, Northern Ireland and Wales all have a four-year term. Local authorities are elected on a four-year cycle. It would make sense to have four years for Westminster also, then an election for one body need never clash with that for another.