I am against the transitional phase as an insane control mechanism on free activity. But I can't say I agree at all with your phrasing of "the other side" of the debate which I would therefore "support."
It's not that people would go straight to communism immediately AFTER expropriating the means of production, but immediately IN expropriating the means of production (in addition to other activity--see #3 below), to the extent that that activity takes the form of communization.
Also, to clarify, the immediacy of communization:
1. is not necessarily temporally immediate. It could take place "over time," just not through the kind of managed, measured, progressive capitalist time we are accustomed to. It is immediate in the sense that the ends are not separated from the means through a mediating body such as party control or the disgusting society of coercion and control which you are describing as socialism.
2. is a theorization of actual forms of class struggle arising in recent decades. In other words, it is not a theory from which practice is meant to be modeled (which would be programmatic and therefore counter to the theory's content), but an understanding of the way in which class struggle has developed (not necessarily progressively).
3. is not limited to expropriation of the means of production, having as its scope the whole transformation of social activity, the expropriation and destruction of commodities and retail locations, the blockade and destruction of various infrastructural systems, the occupation of a variety of spaces, the stoppage and destruction of the means of production, and more that cannot be foreseen. In short, it is the production of communist social relations immediately in and through the destruction of capitalist social relations and their materiality. Better yet, we can't really know what it is except by doing it.
Following #2, it is important that we understand communization not simply as an "issue" that we have to "take a position on." It is undeniably the form which insurrections-of-communist-potential take within/against/beyond the present conditions, and in the course of these uprisings overcoming their limits, they sometimes have and will have to deny and attack the proponents of transitional phases and planning.
Personally, and to depart from the theory itself, I find it intensely repulsive that communists would act as social planners in a mass uprising, trying to plan activity, place limits, force production, and so on. To wax poetic: the flower of communism must be quenched with the blood of these communists in order to truly flourish.
To put it another way: get with the program, which is anything but a program.