Repitching is great, but pitching on a yeast cake is usually a bad idea.
Pros (compared to other repitching methods):
-Fast and easy
-Least risk from a sanitation perspective
Cons:
-Easy to overpitch and destroy ester character
-High percentage of dead yeast can increase risk of autolysis
-Overpitching plus other compounds can increase fusels
-Residual trub plus trub from new wort can increase staling
-Residual hops and other flavor compounds can lend off-flavors, or carry over potentially unwanted flavors
Point is, it's not advisable from a beer flavor and stability standpoint. And these dwarf the sanitation risks compared to other methods assuming your sanitation is up to snuff (and if it's not, you shouldn't be repitching anyway since you'll just multiply whatever bad guys are already there)
Now, there are a few cases where pitching on a whole yeast cake may not be a bad idea- going from a small beer like an Ordinary Bitter to a behemoth like a Barleywine or RIS that needs that big of a pitch rate. But for an average batch, it's not a good idea.
Basically, if you want to save money on yeast, try repitching properly. A lot of folks like to go the "overbuilt starter" method, where you make starters larger than you need, and save the extra yeast for later. I don't personally do it, but there's nothing wrong with it. I use a mix of top cropping (harvesting from the kräusen early on in fermentation, very fast and easy, but only works with some yeast strains) and rinsing out the yeast cake. There's threads on how to do it on HBT, but it basically involves using boiled and cooled water to separate good healthy yeast from trub/hops/dead yeast, as well as allowing you to then measure how much yeast you have, and pitch appropriately, mitigating all the issues from above. The only downside is that when doing this, your cleaning/sanitation must be absolutely impeccable, and it takes a little bit of time.