PIN diodes, when biased on, still have some residual resistance. You want this resistance to be as low as possible to get the maximum attenuation per shunt diode. Starting at around 5 mA per diode they become useable, but with you wanting 70 dB of isolation, I would give them 15 mA each. High power switches would require even more current.
Do a quick analysis:
take a 50 ohm line and put a 5 ohm resistor to ground along the line. Analyze the insertion loss. Now make the resistor 1 ohm and repeat analysis--a big difference in dB's! Also, similarly, you have to worry about any parasitics in the PIN diode, such as series inductance to ground--too much and you do not get much isolation per diode.