Mixed martial arts is a popular form of practice these days. Whilst senior practitioners of martial arts have always dabbled in other martial arts it was until the popularity of cage fighting emerged. Cage fighting was made internationally popular by the UFC (Ultimate Fighting Competition) series in the US. In this competition there are few rules. The competition was initially dominated by the Gracie JuJitsu schools who specialise in ground fighting. Once taken to the ground many a boxer or striking arts practitioner is deprived of their weapons and soon choked out.
Critics of the MMA say that going to the ground is a great strategy for a one-on-one bout in a cage but maybe not so useful in a bar room brawl, yet others say that the mat is padded and a take-down would end the fight immediately as the ground hits pretty hard.
One thing is clear though: the boxers, karateka etc. soon picked up some ground fighting skills and began to reclaim the cage.
MMA schools tend to be grounded in a particular discipline and pick up the others along the way, in the manner that the instructor might have learnt the MMA. Many think that it is important to obtain sufficient skills in a particular art before trying pick up the skills in another art. It all depends on what you are looking for.
Over the years many stylists from other arts have come to Aikido to pick up some additional skills. Is it because Aikido is tougher than other martial arts? In all probability not, as Aikido takes a lifetime to master. What it is I think is the sensitivity training that Aikido provides and a way to work with your opponent through space and time (ma-ai) that can give the stylist an advantage or alternative view point in their own art.