It is true that paystubs are not necessary, but they can help if situation is more complicated than a simple "hired on (date) and worked full time since then". For example, my wife's hours change with each university semester that she teaches, and range from 15h to 45h per week, depending on the semester. You bet we provided a clear breakdown of all 2 years of her experience (which is in reality almost 3 years because of part-time work), even though hours worked are also mentioned in both of her employment letters (a letter from her supervisor and a general HR letter). We made a table going from month to month on hours worked per week in that month and total hours worked in the month, all proven through the paystubs.
Perhaps it was too much, but I know it was difficult for us to calculate when she met the hours (because they change so often), and she is paid monthly vs weekly. We wanted to make the situation as clear as possible for the immigration officer. Obviously, this is a very different situation to someone who worked consistent full-time hours in the past two years (so, no time under 30 hours per week, no gaps, etc.)
It is important to adjust documents to your own situation, but the burden of proof is always on you. Personally, if I had any weeks under 30 hours, I would make sure to have additional weeks to meet the hour requirement (3120 hours). It's also good to be conservative and have added weeks if possible before applying, just in case.