Jason brings up some very good points. If your profitable and are in business for a number of years it gives you the ability to use those profits to make more profits later on. What I mean by this is rent what you need to now, and when the time comes you can start to buy the equipment that your currently renting. As time goes on, you'll have more money to use to buy the things you need and you'll rent less.
Sure it doesn't make sense to buy a stump grinder if all your doing is 5% on stumps. But when you have all the equipment you need, and your accountant tells you that you need to buy something...that is the time to get that grinder...even if its sits for a few days a week. Once you own it I am willing to be you'll start to sell it.
Also, when you have that grinder on site, and do not have to go back to grind...You have your whole crew available to help with clean-up. Thats three or four guys and that can make a half dozen stumps go by pretty quick. (Thanks Jason for that advice)