Are you talking about a beachead as in a remote planet you capture for yourself but don't capture surrounding planets to make it a new "cluster"?
Or are you talking about setting up a base of operations while the AI command station is still there.
If it is the former, I use this strategy all the time, especially in the late game when I need to prepare my armies for homeworld assualts.
If it is the latter,
There was a big discussion a while back about defense, and the viability of beacheads were a part of this (see
http://www.arcengames.com/forums/index.php/topic,10143.0.html). Basically, the inability for turrets and forcefields to work outside of supply really hurts this. However, no one has thought of a way to allow this without it becoming abusable, annoying, or too complicated (a few ideas were tossed around the previous thread, you can read up on those)
Also, many good players do not let the AI's defenses build up enough such that you need a beachead. That, and the AI has a tendency to focus almost all of its reinforcements on alerted planets, leaving non-alerted, non-core, non-homeworld planets pretty pathetic and thus easy to "park" a fleet there without a beachead.
And finally, a single chokepoint defense model is so attractive that it can be hard to convince players to dedicate a few turrets and forcefields to use on an offensive beachead.
So yea, beacheads work, but in the current balance, it is rarely needed until the late game.
If you consider bringing along and occasionally parking in hostile territory a neinzul encalve starship + mobile repairers + cloaker starships to be a beachead, well, I use that all the time too.