well lets start by saying some of the funnest guns that wer made back in theday ran on hpa settups (search youtube for youth engineering mp5 im on ky phone so the link i post wont work for most)
also search google for "airsoft escort system"
many of the "classic" airsoft guns ran off of an external air source
that said the usefullness would be firstly shot to shot consistancy that you just cant really get out of a gas in mag setup, you can fine tune the fps you want and can expect almost every shot to fall prety close to that fps range (great for snipers and designated marksmen who need their rifle to perform the same from first shot to last when accuracy and consistancy is key.
the ability to play in colder weather without freeze up and slow cycling and miss firing and venting of gas from mag like happens with gbb/gbbr's when its cold outside.
yes you would be stuck with a remote line and air tank, but you can get prety small tanks and can get creative with the line and run it along a sling or something.
there is actually a cpl old threads on other forums of bolt action rifles that have the air tank in the stock so there is no remote line sticking out.
the design of todays gbbr's and bolt action gas in mag rifles would require drill and tapping of the fill valve on the magazines with a qd connection to attach the remote line to, the mag would still hold gas and the system would still work the same way just there would be an external air source connected to the mag.
the kjw m700 is probably the most comonly converted "modern" airsoft gun to hpa because gas systems kind of suck for snipers but once the consistancy of gas in mag is adressed via an external setup the viability of the gas powered bolt action becomes alot grater.
i wish i had picked up a YE mp5 back in the day when there will still some floating around but i was a lil poor back then from buying stupidly priced "black market" aeg's befor the laws came into place.
id love to see how theese new gbbr systems respond to hpa setups so have at er and post up lol.
there are still some places to buy classic guns (classicairsoft.net i think)
and also some good wright ups on how to build a hpa kit if you google around