The greatest concern with a lack of exclusivity of titles does not lie with the sorts of complaints that you usually hear. The thing that people should REALLY worry about is that multi-platform releases lead to a slew of issues for those games. The easiest to point out is the issue of development resources being split between several platforms (meaning either a bigger budget or a lower-quality offering).
As a side-effect of the budget issues, the games also tend to be low on the originality and creativity sides, because the developers have a much bigger sum to earn back from their game and thus don't want to take a risk and make the game stand out from the rest (potentially resulting in lost sales if the public doesn't take to it). Meaning that multi-platform releases tend to feel very same-y, familiar, and safe. Often playable, sometimes fun, but nothing we haven't seen before a dozen times at least.
An over-saturation of unoriginal games is not necessarily an issue for a system as long as there are a few stand-out titles that really draw people in for their uniqueness and reassure them that the platform is not heading down the path of mediocrity. But of course, if everything goes non-exclusive, the odds of a title fitting that bill grow smaller and smaller. When the market gets bad enough that there are no creative titles at all on the horizon, there comes a risk of a market crash due to waning interest of consumers at large who get tired of buying rehashes of the exact same games.
That, not the juvenile "my system has more games than your system" argument, is why consumers should be wary of non-exclusivity.
