Whether you're talking about a fighting game or talking about a sport, competition is the most important goal of that sport in order to make it effective and accepted.
For competition to exist, you have your opponents pit their skills against one another to determine who the more skillful opponent is. In Tennis, for instance, two players hit the ball within a bounds to try and get their opponent to miss. They have similar skills, the same equipment and the same rules to work with - that's a competition.
While Smash Bros. and Tennis are very different, a spike is a similar technique in both games. Spikes can be avoided entirely before hand by a skillful player, but if a spike gets set up, it's very likely the victim player will immediately lose a point. A player inexperienced to either game will immediately deem the concept of a spike as "lame" "cheap" or "unfair". But once that player is educated on how they could have avoided the spike and how situational it is to perform, that player realizes it's a unique and important aspect of the game that anyone can prevent or utilize.
But when we start to introduce aspects to a game where the other opponent can't react or can't do anything to prevent a scenario, the competitiveness of the game is lost. Let's take a grab infinite, for example. A player is bound to be vulnerable at some time of play - because of how long it takes to knock a stock off of an opponent, players will be put in a position where they can be grabbed. If "Don't get hit" had any merit, you'd be able to find a video of someone getting 3 stocked without the other taking any damage (but you can't). With no way to prevent a grab and nothing to do once you are being infinited, the competitive aspects of the game are abolished.
And that's really the most important difference between a spike (gimp, ledgehog, or any other technique) and an infinite, chain grab, ledge stall, etc. One can be prevented while the other cannot.
While everyone plays to win, there isn't a game if there isn't competition. If playing to win is all that matters to a person, then where is the line draw between an exploit and cheating? Whether it's within the game's code or not shouldn't matter - all that should matter is if the scenario is preventable or counterable by an opponent. If it's unreasonable, a rule is formed to prevent that event. That retains competition and a test of skills between players.
The problem with a lot of people here is that they don't want to bother with rules because they make it hard to monitor the game and make sure those rules are enforced. I think we all want to lynch the people responsible for screwing up Brawl so badly, but the best thing we can do is make up rules to prevent anti-competitive glitches and mechanics in the game. And if "playing to win" is all that matters and "play to compete" is ignored, then get used to a game that's as lackluster as rock-paper-scissors, checkers and tic-tac-toe. We all know how popular those games are.