They vary. Typically, the issues are availability and cost.
For normal bookings, you can't guarantee that it (or anything else nearby) is going to be free when you have that booking.
For 'touring', most have a price structure that heavily favours short 'there and back' journeys, rather than taking it away for a few days. If you could leave it somewhere other than its base, it'd be easier because you're not paying for the time spent in the hotel or wherever.
The one that has a car space 20m from where I am at the moment wants ?54 a day or ?6 an hour, plus ?59.50 a year. Compared to that, the one you mention is a bargain, especially for its 'very useful for late outcalls' overnight rate. If you are sure of getting a couple of such bookings, it'd be worth considering booking it in advance to avoid the first issue.