I think the US Marines are used as much as they are because of traditional roles and power in the US Federal government. I think the President has more authority to direct the Navy (thus Marines) to combat situations where there is no formal declaration of war. With all the small conflicts and situations where there is no formal declaration against a formal nation its easier to simply send in the Marines than the regular Army.
Special forces are indeed useful and actually do exist and do have a purpose. The problem we see is that what once was a small group of the elite, best picked, best trained soldiers and police with special roles and purposes grow into something they are not.
As our German author stated in the other reply left here, if the special forces get too big, the drain of talented soldiers from the main army weakens the rest of the army by the absence of high quality soldiers bolstering units and being NCO's to lead squads and sections.
The other problem is degradation of the special units themselves due to size. The smaller the special forces units are, the less of them there are, the more critical you can be and selective you can be, only taking the VERY best of the best. SEAL's, SAS, ect., can be very high quality because there are so few of them
Once the special forces grow to too large a size there is severe and wide degradation of special forces as the only way to grow the number of special troops is to simply reduce standards to the point those standards become the same as regular troops, or closer too, or close to. Forming a crack 15,000 man division might yield a great, high grade regular standing army division a cut above the rest, but they will be not as good as SAS because they have no choice but to take men that are even a little lower. Raising a few hundred of the best of your nation yields you pure gold, raising many thousands, tens of thousands, means you will have to take a lot of lead and silver to fill out the molds, so to speak. You have an elite field division, great for fighting regular war in the field, but not quite a small elite special forces unit meant for special operations and missions.
Thus, why the 'special forces" meme comes about, "speical needs forces" because at some point if you water down the special forces they aren't special. You take a non elite unit and throw it into elite unit style operations, missions, roles, they will utterly fail and you will get a laughing stock. A high grade infantry unit full of veterans can do a lot of things, but to do what a special unit that is expert and trains nonstop for a single role like hostage rescue as well as those heavily specialized and overtrained is a good discussion to have.