The bachelor is a strange testament to the decline of our society. People are that desperate for fame and attention that they would pretend to find love on tv. Some of them even have relationships already at home. I roll my eyes every time I accidentally turn it on and move to channel surf as quickly as my little fingers would allow. Now, having a celebrity on (and no I don't mean Flavor Flav), I would definitely watch if they are attractive and truly trying to find love.
The show is a bit silly and is not really real. There is no reason why celebrities can not compete to win. Few bachelor couples every really fall in love and if its fun and provides a rating boost, there is no reason why they cannot have a little fun.
Yes, celebrities should be allowed on bachelor. If a celebrity is single, there is no reason that the celebrity should be excluded from the bachelor. It may be more fun to watch celebrities compete on the show. This may attract a larger audience who wants to see if the celebrity gets chosen.
From a business standpoint this makes sense; the presence of celebrities drives ratings significantly higher, in turn bringing in a lot more money. Viewers who are not happy with the chosen contestants are free to switch their television to an alternative program; no one is forcing them to watch the show.