A US or a Philippines passport holder enter THailand. Both countries are in the 30 NO visa needed list.

And they stay in Pattaya. Can thy just continue crossing the boarder into Cambodia every month and re enter, extending their stays for another 30 day?  Can this be done every month for 6 to 9 months?  Is it OK / Legal??  Any problems doing this???

For US passport this should not be a problem, but could be for Filipino passport holder.

Why not get double entry Tourist visas or even Non-Immigrant visas instead? Then you only need to cross the border every 90 days.

In past I never stayed in Thailand for longer than 60 days, so I dont know exactly how it works to get the most out of double entry tourist visa.  My understanding is you get 60 day stamp with this at airport, get it extended for another 30 days at immigration office in Silom,  and than do border run come back in on same visa and get stamped again for 60 day again. And than get an extension again at immigration for 30day. And, that is how to get the most out of a Tourist double entry. Does this sound about right? Also do they automatically give you a 60 day entry during the visa run, if you are on a double entry??  Thanks in Advance!  ???