The MX-500 can be told to punch through volume or channel functions of another device just like the 950 remote. For that matter you can do that on the URC remotes too, but with the computer you can do all the punch throughs in one fell swoop, saving your weary thumbs from all that poking. Not really a big advantage, but really time saving (after the hours spent figuring out how to do it ).
Like I mentioned, the coolest thing with the JP1 is the ability to get those discrete codes such as power-on and power-off, and for my Sony HS10 HDTV, discrete codes for the different video inputs (e.g. video 5 for component input). So, with the ability of the MX-500 to store up to 15 macros (of up to 20 steps each), you should have a lot of flexibility to configure a macro for all of your devices and preferred audio setups.
For the budget minded, the URC8811/JP1 combo ($40) is really flexible and could serve as an acceptable and economical substitute for the 950 remote. It can take care of one peeve several have with the 950 remote: turning on the 950 with the punch of any of the device buttons. Now you can have just one remote that's reasonably intuitive and can be programmed or taught all the codes of pretty much any IR device. The disadvantage with the 8811 is that you can't edit the button labels like you can on the MX-500.

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Home theater: the hobby the whole family can enjoy - whether they want to or not