Just got a set of Porterfield R-4S brake pads (AP430/AP540 front/rear) to replace the OEM pads on my 2K.
The Porterfields are a bit different from the OEMs. There is no metal tab on top of the pad which is present on the OEMs. This tab gets fastened by a clip attached to the caliper.
This picture http://www.pdm-racing.com/specials/imag/kvrpads2.jpg has tabs similar to Maxima OEM's.

Is this tab important? Should it be present? R-4S seems to be a popular choice for Maxima on this forum, there should be an easy answer, right?

 

Those are the wear indicators. they are the part that starts to squeak and squeal before your pads completely wear out.

It's fine to use the pads, as long as you keep an eye on the pad wear. every month or so, pull a wheel off and look at ALL the pads. not just one pads on each caliper, look at both pads on all calipers. some will wear faster than others (why, I can't tell you, but they do).
just make sure you replace them before they go metal-to-metal and you'll be fine.

 

Sorry, the illustration was misleading. I do not think these are wear indicators, although the wear indicators are also missing. Here is a photo of my caliper with the "tab" in question: http://liquidhome.com/~lvi/brakes/brake.jpg

The metal tab is attached with a rivet to the pad, the metal spring running through it is a part of a clip attached to the caliper. If I understand it right, the purpose of the spring is to pull the pad from the disk when brakes are not applied.