Actually, you do own the cards. That's a common misconception.
WotC gives you access to their SERVERS.
And quite frankly, unless you're stupid enough to act up a dozen times in a row on the same offense and get yourself banned, that's the same thing as owning them in real life. There are other advantages too...
1) I can play Magic whenever I want.
2) Getting a draft together in RL: Difficult. Getting a draft together on MOL: Instantaneous.
3) I never need to own more than 4 copies of one card. Ever.
4) Local trading partners: 10. MTGO trading partners: 10,000+.
5) I can't lose my digital cards.
6) My digital cards are always in mint condition.
7) There is no shipping involved in trading my digital cards.
8) My digital cards don't weigh an ounce.
9) MTGO is the same as having a level 5 judge watching you at all times. (Imperfect, but nearly so.)
10) I don't have to see/smell/hear my digital opponents.
11) Block button. Block button. BLOCK BUTTON.
12) No paying $1.50 for a can of Coke on MOL.
13) My MOL cards don't need sleeves, deck boxes, longboxes, or toploaders.
14) Getting a league together in RL: Almost impossible. Getting a league together on MOL: Instantaneous.
15) MOL has a room for newbies and a room for multiplayer. RL seldom does.
16) Opponents can't cheat during MTGO games.
17) Randomization really is random in MTGO games.
18) If I have a question, there are always Adepts online.
19) Privately chatting with someone in RL is inconvenient; in MTGO, it's easy.
20) A much larger player pool allows an actual economy to flourish on MTGO; the phrase "well, that's what it's worth to me" is almost never heard.
There are disadvantages too (the occasional lag, the very occasional server crash), but I think the advantages outweigh them.