Define "theoretically valid". They do leave someone with the book-knowledge we demand of rabbis, and therefore someone with online semichah should not feel like a charlatan if he expects people to call him "rabbi". For that matter, someone whom rabbis feel is knowledgeable in other ways and should be in a religious leadership role may be declared a "rav umanhig -- a rabbi and leader"; dubbed "rabbi" without knowing the usual material for semichah.
However, to be a community's poseiq (halachic decisor), you need apprenticeship. There is an art to pesaq (decision-making), aside from the book knowledge, that you can only pick up by serving under a mentor and absorbing methodology culturally. We rule as per the gemara (Sotah 21b-22a) which discusses who is the verse's "רשע ערום -- wicked trickser" who is listed among those who cause the world to be swallowed up. Among the suggestions:
עולא אמר: זה שקרא ושנה ולא שימש תלמידי חכמים
אתמר קרא ושנה ולא שימש ת"ח
ר' אלעזר אומר: הרי זה עם הארץ.
ר' שמואל בר נחמני אמר הרי זה בור
ר' ינאי אומר ה"ז כותי
Ulla said: This is one who studies and reviews, but doesn't serve sages.
It was said, one who studies and reviews, but doesn't serve sages:
Rabbi Eliezer said: He is an ignoramus. [Meaning: he isn't to be considered a member of the Pharasaic / Rabbinic class.]
Rabbi Shemu'el bar Nachmeini said: He is a boor [implying more coarseness than a mere ignoramus].
Rabbi Yanai said: He is [like a] Samaritan.
Similar to another gemara (Berakhos 47b) which states:
תניא אחרים אומרים: אפילו קרא ושנה ולא שימש תלמידי חכמים, הרי זה עם הארץ. אמר רב הונא הלכה כאחרים
It was repeated (in a beraisa), "others say" [ie Rabbi Meir said]: Even if someone studied and reviewed, but did not serve sages, that one is an ignoramus.
Rav Huna said: the law is like "others".
So, someone with an on-line semichah without apprenticeship apparently shouldn't be giving halachic rulings, as it risks being classed an evil trickster whose decisions bring ruin. (I say "apparently" because I myself do not feel sufficiently apprenticed to say something firm that could be taken as a ruling!)