My 2p from dodging an awful lot of kelp around these parts...
On an aux engine it is to be avoided at all costs as you rapidly end up motionless and swearing.
On a decent sized main engine it only clogs the prop at idle or very low revs and just gets chopped at high revs. If you stop with the motor in a clump then there can be problems with stalling when putting the engine in gear again. Kelp around the leg can cause cavitation problems and block the water inlets as already said. I've also had problems with it blocking the telltale in my aux.
In my experience the best thing to do is to trim the motor out as far as you reasonably can and keep a bit of speed up, and the kelp or weed will usually just slide off the bottom of the leg without catching. You can plough through quite thick kelp like this but I always try and avoid it if I can, and when in clear water give it a good burst astern to make sure everything has gone.
As also said, kelp tends to attach itself to rocks and other inconvenient solid objects so is more often than not best avoided as an early warning of expensive clanging noises ahead
having said that there is kelp around here which is on the surface and attached to the bottom in 30-40ft of water in quite a few places, so it's not necessarily about to become bumpy.
If you get a good sized clump that is securely attached it's quite handy to tie up to as well