I imagine you don't have to do the scraping server-side if you want to serve the content from there. It's a bit more complex and you have to make sure you don't get fed malicious data, but if your users are willing to put your code on their PC, why not use those resources?
they server can use proxies, if it wants it can use 3-4 different IPs and cross reference the stats to make sure the data is the same, if not, average it, whatever.
Definitely beats 1000 users scraping the same keywords. Also, many SEOs (me included), get OCD over ranks checking the same rank several times a day. There is no need for that, but truth is the same query could be done 10-15 times a day by the same user.