No, im not a doctors receptionist and yes, some of them (thankfully the minority) are bitches. But on the whole they do a difficult job and it really irks me that the general attitude is that they are the dragon on the end of the phone.
I have worked as a veterinary receptionist, which whilst quite different had similar issues.
I was expected to make a decision based on what the client told me whether an urgent appointment would be offered. If i client SAID it was urgent, then it was treated as such and the first available appointment offered. It was helpful to try and get as much information as possible to decide what appointment was the most appropriate. I often used to say to clients "lets see fluffy today" as some people don't like to push for appointments so i would always try and make sure those cases were seen. I wasn't trying to be an armchair vet!
The thing is, there are only so many hours in a day and I was not trying to be difficult if i refused an appointment right there and then - I just couldnt do it, or more often than not id have the vet standing over me making throat cutting gestures to warn me not to make anymore appointments that day as they had an urgent meeting golf to attend to after they had finished their consulting times. Id get it in the neck from the clients and then when the vet finally did condescend to seeing them, they would be oh so polite and "thankyou for seeing me, we tried to get an appointment yesterday but your receptionist woudlnt allow it" 
I can't help but wonder if this happens to Drs receptionists too, more so in small practices i imagine.
Generally our clients were lovely, but some people could be so rude, ranging from condescending to downright abusive, I was called a cunt on more than one occasion - I wasn't i was actually very well liked by the decent sane clients. I'd feel terrible having to refuse people but what can I do if the vet is refusing to see peoples pets or there simply WASN'T a space and i already had a waiting room full of people waiting hours to be Fitted in. The very same people who would complain about being kept waiting because they insisted on being seen straight away!
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On the other hand:
I think that the current system of having to ring for an "urgent" appointment or "emergency" is pants, you try and make an appointment and the receptionist will offer one in three weeks time, but then counter it by saying, "if you ring in the morning and make an urgent appointment you will get seen tomorrow if you managed to get through on time" So there you are 8am, usually whilst trying to get the kids ready for school trying to make an "urgent" appointment, which ok, isnt life or death, but really needs to be seen in the next few days rather than few weeks! This is not the receptionists fault though! This need to change.
I do often find myself having to be very firm with the receptionists at my mums surgery, as unless you persist you will not be given an appointment. Id much rather be asked what the problem is and an appropriate solution be offered, but Drs receptionists don't do this. So its very black and white, theres either an appointment or there isn't. This isnt't the receptionists fault.