The problem is for big crawls( and 500k is not large) you probably don't want to use your desktop for example my home adsl is only 3.5 as we are 6kyards from the exchange.

And I would not want to get my works 100Mb banned by google. This is where services like deep crawl come in to play I can set up my sites to be crawled at night and look at the reports in the morning.

And another problem I found is desktop crawlers are very resource hungry at one small agency we had two striped down dedicated machines just to run crawls as the risk of causing a crash was to high