I mean if one plugin will [download and] copy other plugin in apropriate folder or make a link file, can it dynamically integrate it with platform without restart?
So, it looks like it is possible to build an always-up server, that evolves in time, without being restarted. Is it a demanded(not sure about this word, English isn't my native) feature? I mean does buisness need it or it is easier and ok to reboot? Another benefit I see is an old-version compartibility...
Sure. Using the OSGi console you can (without restarting anything) install new bundles using the "install file:..." command and start them using "start ".
Usually the installing and starting of new bundles without restarting the platform is a great way to showcase the new possibilities of OSGi.
But in my opinion you shouldn't use it like this in production environments because it's not easy to develop your application in a way that make this process reliable.
Of course it is usually no problem to add new functionality via new bundles to a running OSGi application without restarting. But it is a very different situation if you really need a "always-up server", because of the dependencies your bundles have with each other, which makes dynamically changing loaded bundles kind of hard. It is not impossible, but I wouldn't count on it.
So yes, it is easier and ok to reboot when you deploy changes to your application.