Actually it's not quite like that. I have experience with these.
If he's the type to prefer longer sessions (clock-free dates not including overnight, usually a 5-6 hour thing), then he gets a couple of those a month. 2 or 3 which adds up to what my monthly arrangement fee is = $6000/month.
If he's the type to prefer shorter arrangements, 1-3 hours, then obviously he is free to see me up to a certain point that doesn't exceed the monthly fee by a great deal. I wouldn't nitpick an hour or two.
Usually gentlemen who prefer these types of arrangements aren't looking to call you up every day and demand that you are available on a whim. It is still treated like a regular session, with respect to both parties' availability. Also expectation with regards to other partners definitely varies. With my experience, I've found that they preferred 1 or 2 multi hour sessions a week and one or 2 dates that month, usually to a work party or some sporting event.
What people should remember is that with monthly arrangements, respect is still a very important factor. It is more likely to succeed with people you have great chemistry and history with.
The pros? Revenue with a regular client you hopefully enjoy being with. Absolutely won't work any other way. You can't pretend for that long...at least I don't believe so.
Cons? If you aren't used to what this type of thing demands, it can be hard.
A girl who is used to shorter sessions may decide it's not for her versus one who prefers longer ones and enjoys her time.
As well as one who prefers a gift every time versus one who doesn't mind a lump sum at the end of the month. The latter SP will usually succeed with these arrangements.
Then there is the issue of someone who wants you to be exclusive only to him.
If that comes into play, you have to factor in your fees depending on how much you generate monthly or quarterly and how much you expect to lose if you become strictly exclusive. Factoring how long this thing will potentially last, a month or 4 or 6, etc, may end up to be more costly than a normal monthly one (some months/quarters tend to be more lucrative than others).