I'm not saying DS1302 is harder to code for. In fact coding from scratch, bit-banging SPI is generally easier than bit-banging I2C. However you don't want to write your own code for it, you want to find a library to handle it, and due to the bidirectional data pin, there are some issues interfacing with the PIC MSSP in SPI mode, and it will use up four pins whereas an I2C interface RTC only needs two pins and can share them with many other I2C devices.
The 24C32 is just EEPROM memory. Why they bundle it, I don't know. Obviously some application originally needed it and many cheap module sellers are now copying that.
I haven't got the foggiest why anyone would want to put a DS18B20 on the same board. If it had an unpopulated footprint for an I2C temperature sensor it would make more sense.