Well, if it's in the rules and the arena design allows it, it's a legitimate tactic, simple as that. To stay in the fight a robot has to able to move- some rule sets define a particular minimum distance from the robots base radius- and remain on the 'playing surface' too. If the arena design allows you to remove your opponent from the defined arena surface, either OOTA or down a pit for example, that's a knockout unless they can get back into the arena and move about within a defined time period.

Some arenas, like the Battlebox, don't allow OOTAs as such, but robots can still get caught up on arena hazards or stuck against the wall.
Some kinds of arenas favour flippers, enclosed arenas probably favour spinners. It can also be argued that a non-flat surface is a good idea to discourage wedges and flippers, and some arenas do this. Some people think that this would make fights more interesting, some think it wouldn't because they like flippers and rambots. Tastes differ... Builders build to suit the conditions they fight under- in different arenas, different types of machine are more successful.

In the UK scene at the moment there are practical constraints imposed by the need for portable arenas and what Event Organisers consider to be a reasonable size (and therefore cost) to transport, which include not being able to safely run heavyweight spinners. In the absence of spinners, allowing some possibility of OOTAs is good for the audience, as it can be quite exciting and allow a decisive win. Axebots are still quite successful in the UK and there are more of these now that a couple of years back. There are other issues. If it was possible to run HW spinners, fewer events would probably happen each year because builders would be unable to sustain the cost of potentially rebuilding several times a year. Also, builders who have developed flippers for several years and who have a great deal of money put into the hardware are understandably reluctant (or unable) to build completely new machines when rules or arenas change.

All that said, look elsewhere on the forum for discussion on a possible new 'safe' class of heavyweight spinners proposed by Roaming Robots. In an ideal world most builders would like spinners at some events at least. The good news is that there is at least full combat for featherweights once a year, which is really where the cutting edge of the sport is in the UK now.