I have been getting this reaction all over the place since I began using drop in resolution mechanics in my newer games. Not from people who have bought or read the games, strangely enough, but from people who have seen mention of this practice, in forum posts or reviews. I get this question from graybearded grognards and hippy indies. The trad guys don't like the idea of there being multiple choices of mechanics because it's traditional to go with one system, whatever it is. The indie guys don't like the idea because a mechanic should be crafted to fit the theme of the game, and slotting systems in like replacing lightbulbs belittles the SYSTEM MATTERS mantra.
Here's why I do it:
1: SYSTEM MATTERS. Different mechanics have different feels. I like that. I don't try to replicate the feel of the percentile mechanics when I use a dice pool, or when I use diceless. Using a different mechanic produces a totally different game. That's a GOOD THING to me. The price I pay is decoupling the mechanic from the system. Everything has to go through the interface.
2: PEOPLE ARE DIFFERENT. I hear it all the time - "I hate dice pools!" or "I despise percentiles!" or "I much prefer bell curve distribution." So with so many people expressing such extreme views, multiple mechanics give me a better chance of pleasing more people.
3: IT'S FUN AND EASY. I like making up mechanics - I do it in my head while walking, or watching some brainless TV show. It's not rocket science, folks! It's really the easiest part of writing a system. It's also easy to drop in a new mechanic - it really requires just new dice and new character sheets - you can keep the characters.
4: THERE'S LITTLE OVERHEAD. Putting a new mechanic in requires 6-9 pages. That's it. It would require less without examples and with less description, but I'm OK with 6-9 pages.
5: DISTRIBUTION. Getting a new mechanic out to existing customers is as simple as putting a file on the website for download.
6: PUSHING IT DOWN. I prefer to let the group make as many choices as possible, to let them tailor the game for the way they play. This is another way to do this, and fits with my philosophy.