If you think all of something tastes the same, there is a problem with your taste buds, not the product. With that said, American style lagers have their place, same as stouts. Man, it sure is hot, I could go for a nice, heavy, warm stout about now. - said no one ever.
And Guinness is bottom of the barrel as far as stouts are concerned.
OK, lagers do not really all taste the same. German Lagers with good higher mineral well water or stuff that has no adjuncts and decent amounts of hops can be pretty interesting, but, in general, the diversity within the world of lagers is very minimal compared to the range of diversity within the world of ales.
When its hot, I will gladly drink an adjunct lager if a better choice is not available, but, id rather have a wheat ale, a cream ale, or maybe a saison or perhaps a weiss, or, a geuze or a lambic maybe ...
And yes, I realize Guinness is crap as far as stouts go, I tend to prefer imperial, barrel aged, double, coffee, and/or oatmeal stouts, but, I'll gladly enjoy a cheap dry stout any day of the week.