You can extend the initial 60-day entry 30 days at any immigration office for 1900 baht. Then do a border hop for another 60-day entry and repeat. You can even do a border hop on the day before the visa expires and get another 60+30, which means you get nearly 9 months out of the 6-month METV.
No it is not compulsory to do an extension. Border hop with METV at Mae Sai will result in another 60-day entry. Compare w/extension 30 days 1900 baht vs. Mae Sai run from CM by bus (320-638 baht, depending on class) + 500b for the Myanmar border run). So border run is a bit cheaper, but takes a whole day, +60 vs. 30 days.
Technically-speaking it's a bilateral agreement, not a visa exemption, but pretty sure it can be extended same same 30 days (at the discretion of immigration officer, as usual).