Yes, I do have first hand knowledge. I build and repair RPC's to pay for my machining addiction. I have seen a number of them that the potential relay had failed closed and blown the capacitors. The contacts are typically not rated for the amount of current required to start a 3 phase motor off single phase power. If you look at the expected full load start current and then double it that's about what it takes to spin one up. a 15 HP motor can easily draw 90 amps starting because you are forcing it to start in a manner that it's not designed to. The 15 or 20 amp rated contacts in a potential relay are just not suited for that sort of power being run across them and it can over time weld the contacts. That pops the start caps in very short order. There are a couple easy ways to get past it. First is to use the start button (momentary) contacts to close a contactor that controls the start caps. You just hold the start button until the motor spins up. Now this only will work if you have a double momentary open set of contacts on the start button. Of course the other set closes the input contactor.
I actually run three contactors in most of my setups. One controls the input power, it sets between the motor and the single phase power feed. The second is a single or dual pole that controls the start caps. and the third connects to the output and the run caps. That way I am not fighting the L2 to L3 capacitors against the start caps and I always have positive control of the output. I also ALWAYS use pushbutton start that will fail off when the power is interrupted. It's safer that way so I don't have a machine spinning down and all of a sudden restart or have a power failure and the RPC restart when the utility comes back on.
Is the third contactor necessary, probably not. One thing I do with my design is build them to be expandable. Say you wanted a 100HP RPC. There are a couple ways to do that. One is a huge cabinet and 800 amps of start current. It frankly sucks just wiring it because the you need welding cable in the start circuit and connecting it to 1/4 spade lugs, which you can't really do, so you would run a bunch of wires to the multitude of start caps and have a HUGE contactor to deal with the current load. Same thing for the input power feed. The cool thing with RPC's is you can stack them in parallel and they become bigger. so 2 10 HP RPC's in parallel is a 20 HP unit. And you can start a 40 HP idler with a 15 HP RPC if it's built right from the get go. Now 40 is the largest I have ever started with my 15 but it does it without any issue and no motor starter.... just a disconnect in line. SO if you design timer circuits to start in steps, doing a full start on a 15 or 20 HP RPC, you can eliminate all the rest of the start caps for the other motors. Once the first one is started, you connect the second idler motor to it and it spins up, then via a 3 line contactor you connect it's run capacitors, the HP rating has just increased by the size of that idler motor. So a 20, a 30 and a 50 (the 20 and the 30 together start the 50) you reach 100HP load ability with start caps only required for a 20 HP idler. The timing circuits for each individual motor in the string rely on the coupling contactor from the previous idler to apply power to it starting the timer count down. Once the last motor in the string starts and has it's coupling to the output bus in place a final contactor closes and connects the 3 phase power output to the panel for distribution to the connected equipment.