Honestly, I think the best answer is try it, and let us know how it turns out.
People seem to dismiss negative effects from the trub when I talk to them or read the forums. I think the books make more out of it than people have experienced in practice. Supposedly, if it breaks down, it may impart a soapy flavor in the beer. But folks on here seem to typically leave their beers in primary for 3-4 weeks. I've even seen people leaving their beers in primary for 6-8 weeks. And they generally like the resulting beer! That should help lift some concern about trub (although if you left your original beer in primary for those 6-8 weeks, I might proceed a little more cautiously.)
As others have pointed out, you are starting your beer out with way more yeast than you need when you brew on top of a yeast cake. Remember how little yeast your original beer started out with? The yeast cake is many times larger in cell count, and now you are starting a beer with all of that yeast.
I think the primary concern with over pitching is too little yeast characteristics being imparted in the beer. Ester and phenol production is correlated with yeast cell reproduction. Your yeast won't need to reproduce nearly as much since there is already enough of them to ferment your beer when you are fermenting on top of a yeast cake. Depending on the style, you may not care about ester and phenol production. If there are other concerns with over pitching, I'm sure others can chime in.
Is it worth it? I think so. You'll still get the satisfaction of reusing your yeast, improving your overall brewing knowledge and experience. You should already have all the equipment you need, as opposed to yeast washing where you may need to collect some jars and some equipment to make a yeast starter, if you don't already have that.
I just started washing yeast. I haven't yet brewed on top of a yeast cake. I'd like to try it eventually. I'm all for experimenting and having fun. Even if a technique is considered inferior to another, wouldn't it still be fun to try it out and see what results? Try it out, don't worry, relax, have a home brew. (And then be sure to let us know how it turned out!)