when your I485 is pending, you can file work permit, get advanced parole and leave and come back  and don't have to maintain any status because you're in  status for pending appication  , right? so someone who's already in the USA the wait isn't that hard. and it's like 100 times better than being in f1 plus if ur status is still pending and ur spouse becomes citiezn, then u can easliy switch the process at that point/. 
but lets say if the spouse was in Nepal then yes, he/she would have to wait  5 years or so get a visa to come to USA for marriage to a gc holder. so when most suggestions are made saying it takes forever to get benefits from gc holders as spouse and just wait till natrulazation, are those targeted  to people whose spouses are in in other countries and not in the USA. how about the ones who are already in the usa? 
I'm not an expert in immigration matters but just wondering , why wait if you can have wp, advanced parole and don't have to maintain f1 status.
so i guess it all boils down to when the visa becomes available after I-i130 so they can file the I485 and enjoy the wp and advanced parole. so see the visa number availibility and it will give u some good ideas.
http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextchannel=75783e4d77d73210VgnVCM100000082ca60aRCRD&vgnextoid=75783e4d77d73210VgnVCM100000082ca60aRCRD

http://travel.state.gov/visa/bulletin/bulletin_1360.html