This seems a ludicrous idea, particularly if future voting reforms lead to more coalition governments and cross-party working. The proposed system could lead to circumstances in which if the current or any future coalition government were to fail, we coudl end up with an ineffective Government for a prolonged period until the fixed term is at an end, and this could be one, two, three or more years.
The biggest problem as I see it is that elections are won on marginal seats, and no MP representing these seats will ever vote for an election, they will hope to keep their position for as long as possible, rightly or wrongly, particulalry if the Government has done things which are unfavourable in their constituency.
Each MP gives up a career to go to Parliament, and so they are likely to want to get their 5 years worth of time in, because to leave early means they will loose their salary and have to find new work. As we have seen with the expenses debacle, there is a natural greed, so being an MP in an effective Government could become nothing more than a cash-cow for these people.
If the PM has the right to call an election, then he/she is sheltered from this problem because leaders of each party tend to always stand in safe seats, so rather than being influenced by mere greed (or concern for future work), the PM can think of the good of the whole party, and more specifically the country.
I am extremely worried that the system which is designed to give strong Government will give nothing more than a seriously weak Government in some cisrcumstances, and this is exactly the time when we do not need this.