If you seriously concentrate on building a character around summoning they can get very strong indeed.
I built a Master Summoner that had its origin in seeing a build, by Mercadior I think, and thinking "I can improve on this". By methodically adding to your summoned creatures power, even if the improvement is not so large in itself, the strategy gets very powerful.
Eventually statements like this "It is quite easy to overrun the board with weak summoned creatures. The thing is they are weak summoned creatures." seem hopelessly wrong. I would bring out some kind of feline with multiple attacks that had decent chances to hit.
And if you are not playing a master summoner, and my GM comes down with all the symptoms of typhoid if the idea is floated, the strategy can likewise be made very powerful. It is not so easy, but you are a full caster as well, rather than a half caster. Cauldrons are great as are rods of giant summoning.